I did some research about this error and found out it has already been fixed for Greenshot 1.3 (official release has not been published yet). If you are okay with installing an unstable (i.e. not thoroughly tested version), you can get it here: https://getgreenshot.org/version-history/ - otherwise you need to wait until version 1.3 is final and released (no scheduled release date yet).
Software version: 1.2.10.6-RELEASE-c2414cf0149a1475ea00520effc01b40087c225c (64 bit)
.NET runtime version: 4.0.30319.42000+
Time: 2020-10-05 07:54:22 +02:00
OS: unknown (x64) 10.0 build 18362 revision 0
GDI object count: 23
User object count: 18
Exception: System.NullReferenceException
Message: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
Stack:
bei GreenshotImgurPlugin.ImgurPlugin.<UpdateHistoryMenuItem>b__13_0()
bei System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj)
bei System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
bei System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
bei System.Windows.Forms.Control.InvokeMarshaledCallback(ThreadMethodEntry tme)
bei System.Windows.Forms.Control.InvokeMarshaledCallbacks()
Configuration dump: