We are getting another exception while closing/destroying a Panel inside a TabPanel.
This time we got a GridPanel with PagingToolbar, FilterHeader and many columns inside a Panel in a TabPanel.
When the Tab/Panel is closed a JavaScript exception is thrown:
ext.axd?v=4.8.1:8134 Uncaught TypeError: Cannot read property 'apply' of null
    at constructor.aliasOneMember.<computed> [as on] (ext.axd?v=4.8.1:8134)
    at constructor.onLoad (ext.axd?v=4.8.1:12800)
    at Object.elevate (ext.axd?v=4.8.1:1926)
    at timerFn (ext.axd?v=4.8.1:4838)
        createAlias: flexSetter(function(alias, origin) {
            aliasOneMember[alias] = function() {
                return this[origin].apply(this, arguments);
            };
            this.override(aliasOneMember);
            delete aliasOneMember[alias];
        })
With some debugging I was able to find the cause of the problem.
It looks like the store in the GridPanel is destroyed and set to null, which causes the PagingToolbar to refresh the store it is bind to and tries to load new data from the "ext-empty-store" which causes the exception because the store isn't there anymore.

My current workaround is to override the bindStore method of Ext.toolbar.Paging and add a return statement if the store variable is set to "ext-empty-store".

Ext.toolbar.Paging.override({
    bindStore: function(store, initial, propertyName) {
        var isEmpty = store === 'ext-empty-store' && initial;

        if (store && !isEmpty) {
            store = Ext.data.StoreManager.lookup(store);
        }

        if (!store || isEmpty) {
            if (this.ownerCt) {
                store = this.findStore();
            } else {
                store = 'ext-empty-store';
                this.needFindStore = true;

                return;
            }
        }

        this.callParent([store, initial, propertyName]);
    }
});
Currently I'm unable to reproduce this in a sample application, but maybe you are able to find the bug with the informations provided.
It looks like it can be a timing problem, because there are some timer functions involved and the problem disappears if the amount of columns in the GridPanel is reduced.