Hi Adeel,
The older SharePoint Designer version 15.0.4420.1017 did not have the SP1 update installed. If SP1 is installed, the version will be 15.0.4569.1506 or higher. See: description-of-microsoft-sharepoint-designer-2013-service-pack-1-sp1
I used to have the same issue and resolved it by installing Service Pack 1. I think some functionality may not work in the older vesion before sp1.
Here is a similar issue:
workflow-opens-blank-on-sharepoint-designer-2013
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