Fixed throwing an ExternalException to user

ClipboardMonitor.cs

@@ -117,7 +117,17 @@ protected override void WndProc(ref Message m)
 
             private void ClipChanged()
             {
-                IDataObject iData = System.Windows.Forms.Clipboard.GetDataObject();
+                IDataObject iData = null;
+                try
+                {
+                    iData = System.Windows.Forms.Clipboard.GetDataObject();
+                }
+                catch (ExternalException)
+                {
+                    // The internal implements will raise the ExternalExcption
+                    // when the clipboard is being used by the another process.
+                    return;
+                }
 
                 ClipboardFormat? format = null;
 
@@ -231,4 +241,4 @@ public enum ClipboardFormat : byte
         /// <filterpriority>1</filterpriority>
         Serializable,
     }
-}
+}