Index: branches/5.1.x/core/admin_templates/js/jquery/thickbox/thickbox.js
===================================================================
diff -u -N -r13887 -r13895
--- branches/5.1.x/core/admin_templates/js/jquery/thickbox/thickbox.js	(.../thickbox.js)	(revision 13887)
+++ branches/5.1.x/core/admin_templates/js/jquery/thickbox/thickbox.js	(.../thickbox.js)	(revision 13895)
@@ -483,6 +483,11 @@
 }
 
 TB.remove = function ($e, $on_close) {
+	if (TB.useStack && TB.windows.length == 0) {
+		// no windows opened, but TB.remove called -> ignore
+		return ;
+	}
+
 	var $last_window = TB.useStack && TB.windows.length <= 1;
 
 	if ( !$.isFunction($on_close) && $.isFunction(TB.lastParams.onAfterClose) ) {