When dropping a movie with no rom loaded and the user cancels the open rom dialog, don't throw an error message, simply quietly cancel the drop

BizHawk.Client.EmuHawk/MainForm.Events.cs

@@ -2410,6 +2410,11 @@ namespace BizHawk.Client.EmuHawk
 					OpenRom();
 				}
 
+				if (Global.Emulator.IsNull())
+				{
+					return;
+				}
+
 				StartNewMovie(MovieService.Get(filePaths[0]), false);
 			}
 			else if (ext.ToUpper() == ".STATE")
@@ -2440,6 +2445,11 @@ namespace BizHawk.Client.EmuHawk
 					OpenRom();
 				}
 
+				if (Global.Emulator.IsNull())
+				{
+					return;
+				}
+
 				// tries to open a legacy movie format by importing it
 				string errorMsg;
 				string warningMsg;