Failed to create IXPlaceholder

I am consistently getting this error in every project (even those that were working well earlier). I am using Xcode 12, was working on a few projects that were working absolutely fine, then one fine day this error pops up and I can't seem to resolve this. Please help -

This happens when I build the app & launch it on simulator. Also, when I open the storyboard, it says 'An internal error occurred. Editing functionality may be limited'.

Details

Failed to create IXPlaceholder for app bundle ID in.storme.app.STORME-APP
Domain: IXErrorDomain
Code: 2
Failure Reason: Failed to create promise.
User Info: {
  FunctionName = "+[IXPlaceholder _placeholderForBundle:client:withParent:installType:metadata:error:]";
  SourceFileLine = 309;
}