After Firefox start group bar is empty

[mnn]

In browser console is TypeError: contentWindow is null on line 176 of tabgroupbar.js

relevant code:
let groupItems = contentWindow.GroupItems.groupItems; in objTabGroupBar.addGroupTabs

[mnn]

I modified (hacked) that function and it seems to be working. Be warned - consider my solution only as a temporary workaround (I have no deeper understanding of that code, I cannot say if it won't affect some other stuff).

// Puts the tabs on the main bar
objTabGroupBar.addGroupTabs = function() {
    if (typeof this.getTabView === "function") this.tabView = this.getTabView(); // monnef fix
    if (this.tabView == null) return; // monnef fix
    let contentWindow = this.tabView.getContentWindow();
    if (contentWindow == null) return; // monnef fix
    let groupItems = contentWindow.GroupItems.groupItems;
    let activeGroup = contentWindow.GroupItems.getActiveGroupItem();
    for (let i = 0; i < groupItems.length; i++) {
        this.addGroupTab(groupItems[i]);
        groupItems[i].addSubscriber("close", this.reloadGroupTabs);
        this.tabsLoaded = true;
        if (groupItems[i] == activeGroup) {
            this.tabsContainer.selectedItem = this.tabsContainer.lastChild;
        }
    }
};

[mnn]

Alright, my previous code just suppresses showing errors but doesn't actually fix anything. I forked it and tried to make it working (and now it, for most part, is). If creator or anyone else is interested, then here you go -> https://github.com/mnn/Tab-Group-Bar.

[Newbiez]

Please note that the 'this' object at startup points not to the "expected" container (object object). It points to window object in this context.

TypeError: contentWindow is null tabgroupbar.js:1xx
"Preference change" tabgroupbar.js:xx
TypeError: this.addGroupTab is not a function tabgroupbar.js:1xx
"1:[object ChromeWindow]" tabgroupbar.js:1xx

The 'this' reference should be used as rarely as possible. Please use the container object generally instead 'this' to avoid these side effects. Please recheck.

[BrenBarn]

I've taken a few stabs at this bug, but it's very difficult to pin down. I've thought that I fixed it a few times but it popped up again. The behavior isn't reproducible in any straightforward way --- sometimes it fails to load, sometimes it doesn't. This makes it hard to know whether a solution really works or the problem just stopped by coincidence. There may also be some interference from the Tab Mix Plus extension, which apparently also tries to hook into tab group loading.