Index: branches/5.0.x/core/admin_templates/js/jquery/thickbox/thickbox.js
===================================================================
diff -u -N -r12117 -r13373
--- branches/5.0.x/core/admin_templates/js/jquery/thickbox/thickbox.js	(.../thickbox.js)	(revision 12117)
+++ branches/5.0.x/core/admin_templates/js/jquery/thickbox/thickbox.js	(.../thickbox.js)	(revision 13373)
@@ -64,6 +64,15 @@
 TB.show = function (params) {
 	TB.lastParams = params;
 //	caption, url, imageGroup, onDataReceived, onAfterShow, postParams
+
+	if (TB.imgLoader === undefined) {
+		// loader image is set from $(document).ready(...) and when some slow javascript
+		// makes page busy and user clicks on something, that opens window, then there will
+		// be an error
+
+		return ;
+	}
+
 	try {
 		if (TB.useStack) {
 			// increment window counter