System.NullReferenceException at Microsoft.Maui.Platform.KeyboardAutoManagerScroll.AdjustPosition()

Question

System.NullReferenceException at Microsoft.Maui.Platform.KeyboardAutoManagerScroll.AdjustPosition()

Frank Schmitt 5

I'm working on a Maui plugin that sometimes presents some native iOS UI.

I'm seeing a crash at Microsoft.Maui.Platform.KeyboardAutoManagerScroll.AdjustPosition() when that UI is presented modally and a DidUITextBeginEditing notification is fired (by the native UI).

Is there something I can do to tell the Maui layer to pause managing UI/keyboard/scroll stuff?

Is there source code I can look at to get a better idea of what's going on?

Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) 50,126 Reputation points Microsoft External Staff

2024-03-05T06:04:30.86+00:00
For further investigation, could you please provide the following information?

The MAUI version.

Could you please provide a code snippet or reproduction step that caused the crash and update it to the issue?

MAUI does not provide an API for pausing UI threads, you can refer to Create a thread on the .NET MAUI UI thread for more information.

Frank Schmitt 5

Sorry for the delay, but I finally figured out how to reproduce the crash consistently.

It seems like when non-Maui code opens a full-screen modal view controller with a text field (or two, to experience the crash with a hardware keyboard), it causes a crash in the KeyboardAutoManagerScroll.AdjustPosition() method.

Using a different UIModalPresentationStyle (that doesn't completely cover the Maui view controller) avoids the crash.

I was able to reproduce by replacing the contents of MainPage.xaml.cs (in a template project) with the code below.

After building and running, tap the "Click me" button and focus one of the text fields (with "Connect Hardware Keyboard" unchecked) to crash, or with a simulated hardware keyboard, tap into one text field and then the next.

using CoreGraphics;
using UIKit;

namespace keyboard_crash;

public partial class MainPage : ContentPage
{
	public MainPage()
	{
		InitializeComponent();
	}

	private void OnCounterClicked(object sender, EventArgs e)
	{
		var rootVC = UIApplication.SharedApplication.KeyWindow.RootViewController;

		if (rootVC != null) {
			var testVC = new TestViewController();

			var testNC = new UINavigationController(testVC);

			// Changing this from the default to "FullScreen" is the key to making it crash.
			testNC.ModalPresentationStyle = UIModalPresentationStyle.FullScreen;

			rootVC.PresentViewController(testNC, true, null);
		}
	}
}

public class TestViewController: UIViewController {
	UITextField TextField1;
	UITextField TextField2;

	public override void ViewDidLoad() {
		base.ViewDidLoad();

		View.BackgroundColor = UIColor.White;

		TextField1 = new UITextField(new CGRect(20, 120, 200, 20));
		TextField2 = new UITextField(new CGRect(20, 220, 200, 20));

		TextField1.BorderStyle = UITextBorderStyle.RoundedRect;
		TextField2.BorderStyle = UITextBorderStyle.RoundedRect;

		View.AddSubview(TextField1);
		View.AddSubview(TextField2);
	}
}

Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) 50,126 Reputation points Microsoft External Staff

2024-03-12T06:20:53.6033333+00:00

The cause of this issue could be a conflict between two different pop-up methods. Have you tried rootVC.PresentViewController(testVC, true, null);?
Simon 0 Reputation points

2024-03-12T12:13:39.3266667+00:00

Hello, I'm having the same issue. Did you find a solution for this?
Frank Schmitt 5 Reputation points

2024-03-12T18:30:28.24+00:00

@Simon I have found a workaround by using a non-full-screen UIModalPresentationStyle for the presented view controller.
Frank Schmitt 5 Reputation points

2024-03-12T18:34:01.7133333+00:00

@Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) Presenting the view controller without wrapping it in a navigation controller doesn't seem to help. The only workaround I have found is to use something like PageSheet for the ModalPresentationStyle of the VC being presented.

1 answer

Your answer

Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) 50,126 Reputation points Microsoft External Staff

2024-03-05T06:04:30.86+00:00

For further investigation, could you please provide the following information?

The MAUI version.

Could you please provide a code snippet or reproduction step that caused the crash and update it to the issue?

MAUI does not provide an API for pausing UI threads, you can refer to Create a thread on the .NET MAUI UI thread for more information.
Frank Schmitt 5 Reputation points

2024-03-11T18:58:23.7033333+00:00

Sorry for the delay, but I finally figured out how to reproduce the crash consistently.

It seems like when non-Maui code opens a full-screen modal view controller with a text field (or two, to experience the crash with a hardware keyboard), it causes a crash in the KeyboardAutoManagerScroll.AdjustPosition() method.

Using a different UIModalPresentationStyle (that doesn't completely cover the Maui view controller) avoids the crash.

I was able to reproduce by replacing the contents of MainPage.xaml.cs (in a template project) with the code below.

After building and running, tap the "Click me" button and focus one of the text fields (with "Connect Hardware Keyboard" unchecked) to crash, or with a simulated hardware keyboard, tap into one text field and then the next.

using CoreGraphics; using UIKit; namespace keyboard_crash; public partial class MainPage : ContentPage { public MainPage() { InitializeComponent(); } private void OnCounterClicked(object sender, EventArgs e) { var rootVC = UIApplication.SharedApplication.KeyWindow.RootViewController; if (rootVC != null) { var testVC = new TestViewController(); var testNC = new UINavigationController(testVC); // Changing this from the default to "FullScreen" is the key to making it crash. testNC.ModalPresentationStyle = UIModalPresentationStyle.FullScreen; rootVC.PresentViewController(testNC, true, null); } } } public class TestViewController: UIViewController { UITextField TextField1; UITextField TextField2; public override void ViewDidLoad() { base.ViewDidLoad(); View.BackgroundColor = UIColor.White; TextField1 = new UITextField(new CGRect(20, 120, 200, 20)); TextField2 = new UITextField(new CGRect(20, 220, 200, 20)); TextField1.BorderStyle = UITextBorderStyle.RoundedRect; TextField2.BorderStyle = UITextBorderStyle.RoundedRect; View.AddSubview(TextField1); View.AddSubview(TextField2); } }
Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) 50,126 Reputation points Microsoft External Staff

2024-03-12T06:20:53.6033333+00:00

The cause of this issue could be a conflict between two different pop-up methods. Have you tried rootVC.PresentViewController(testVC, true, null);?
Simon 0 Reputation points

2024-03-12T12:13:39.3266667+00:00

Hello, I'm having the same issue. Did you find a solution for this?
Frank Schmitt 5 Reputation points

2024-03-12T18:30:28.24+00:00

@Simon I have found a workaround by using a non-full-screen UIModalPresentationStyle for the presented view controller.
Frank Schmitt 5 Reputation points

2024-03-12T18:34:01.7133333+00:00

@Yonglun Liu (Shanghai Wicresoft Co,.Ltd.) Presenting the view controller without wrapping it in a navigation controller doesn't seem to help. The only workaround I have found is to use something like PageSheet for the ModalPresentationStyle of the VC being presented.

Answer 1

Hello,

I make a sample and reproduce your issue. And I noticed that you raised an issue at GitHub - Null Pointer Crash in KeyboardAutoScrollManager.AdjustPosition · Issue #21160 · dotnet/maui · GitHub, please follow the progress.

Also, you could try the solution:

#if IOS
            KeyboardAutoManagerScroll.Disconnect();
#endif

Best Regards,

Wenyan Zhang

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System.NullReferenceException at Microsoft.Maui.Platform.KeyboardAutoManagerScroll.AdjustPosition()

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