fix race condition in test of active window from input thread (which was a bad idea in the first place) which can throw null references in unlucky cases (fixes #917)

BizHawk.Client.EmuHawk/Input/Input.cs

@@ -414,6 +414,7 @@ namespace BizHawk.Client.EmuHawk
 
 					}
 
+					//WHAT!? WE SHOULD NOT BE SO NAIVELY TOUCHING MAINFORM FROM THE INPUTTHREAD. ITS BUSY RUNNING.
 					bool swallow = !GlobalWin.MainForm.AllowInput(false);
 
 					foreach (var ie in _NewEvents)

BizHawk.Client.EmuHawk/MainForm.cs

@@ -643,15 +643,15 @@ namespace BizHawk.Client.EmuHawk
 
 			// even more special logic for TAStudio:
 			// TODO - implement by event filter in TAStudio
-			if (ActiveForm is TAStudio)
+			var maybeTAStudio = ActiveForm as TAStudio;
+			if (maybeTAStudio != null)
 			{
 				if (yieldAlt)
 				{
 					return false;
 				}
 
-				var ts = (TAStudio)ActiveForm;
-				if (ts.IsInMenuLoop)
+				if (maybeTAStudio.IsInMenuLoop)
 				{
 					return false;
 				}