Ticket #5843: 5843.patch

_source/plugins/selection/plugin.js

@@ -17 +17 @@
                         // In IE, the "selectionchange" event may still get thrown when
                         // releasing the WYSIWYG mode, so we need to check it first.
                         var sel = this.getSelection();
-                        if ( !sel )
+                        if ( !sel || !sel.document.$.window )
                                 return;
 
                         var firstElement = sel.getStartElement();