"Uncaught TypeError: Cannot read property 'document' of null" error when edit custom content pane twice

After digging in CKEditor 3.6.6.1 sources I found location where this error is triggered:

getFrameDocument : function()
		{
			var $ = this.$;

			try
			{
				// In IE, with custom document.domain, it may happen that
				// the iframe is not yet available, resulting in "Access
				// Denied" for the following property access.
				$.contentWindow.document;
			}
			catch ( e )
			{
				// Trick to solve this issue, forcing the iframe to get ready
				// by simply setting its "src" property.
				$.src = $.src;

				// In IE6 though, the above is not enough, so we must pause the
				// execution for a while, giving it time to think.
				if ( CKEDITOR.env.ie && CKEDITOR.env.version < 7 )
				{
					window.showModalDialog(
						'javascript:document.write("' +
							'<script>' +
								'window.setTimeout(' +
									'function(){window.close();}' +
									',50);' +
							'</script>")' );
				}
			}

			return $ && new CKEDITOR.dom.document( $.contentWindow.document );
		},

$ variable has iframe object. But $.contentWindow is NULL. I have no idea why this happens.

I found the reason. Making a patch atm.

This patch adds reaction on the 'CToolsDetachBehaviors' which is triggered when you close Ctools modal form (where Custom content appears). It finds and kills appropriate CKEdtior instance and now it can be relaunched correctly many times.

But this event triggered only when you close modal form using "Close Window" control button (in the upper right corner of the window). When you press "Cancel" button, this even isn't triggered and instead — standard Drupal detach behavior hook is called which doesn't kill the instance when "trigger" parameter equals to "serialize" (which is the case when you click "Cancel" button).

As far as I understand the correct behavior would be to kill CKEditor instance if Cancel is pressed. But I haven't find the way to get the information whether a form was submitted using "Save" or "Cancel" button — it just not available in a detach handler (even if having form object in context parameter). So this vulgar patch most likely is wrong:

diff --git a/includes/ckeditor.utils.js b/includes/ckeditor.utils.js
index 0a2aee8..ccd7c89 100644
--- a/includes/ckeditor.utils.js
+++ b/includes/ckeditor.utils.js
@@ -333,9 +333,9 @@ window.CKEDITOR_BASEPATH = Drupal.settings.ckeditor.editor_path;
     function(context, settings, trigger){
       $(context).find("textarea.ckeditor-mod.ckeditor-processed").each(function () {
         var ta_id=$(this).attr("id");
-        if (CKEDITOR.instances[ta_id])
+        if (CKEDITOR.instances[ta_id]) {
           $('#'+ta_id).val(CKEDITOR.instances[ta_id].getData());
-        if(trigger != 'serialize') {
+        //if(trigger != 'serialize') {
           Drupal.ckeditorOff(ta_id);
           $(this).removeClass('ckeditor-processed');
         }

#5 doesn't work for me.

1) Add custom content
2) Close using Cancel
3) Add again —> no CKEditor loaded.

UPDATED. #6 won't work either.

Yes, I did.

I have no clean setup, as and I'm on Drupal + CKEditor + Panels git verions.
I believe the reason why #6 doesn't work — it doesn't resolve the issue "trigger = serialize" as I described in #4.

How comes its not a problem if CKeditor gets detach with trigger = serialize and not removing its instance? It is not unloaded. Or what is unload event?

Have you tried adding custom content, canceling it, then adding again without closing Add content dialog?