Tilemap Heatmap error - possible cleanup issue

[w33ble]

If you add a tilemap with a heatmap to the dashboard, you'll see an error in the console when the saving the dashboard or leaving the dashboard app. It does not seem to affect the actual rendering of the heatmap though, you only see the error if you happen to have the console open.

This affects 4.4.x for sure, but may be a problem in earlier builds as well.

I suspect that given the "width is 0" message that it's a problem with the cleanup/destroy handler, but I haven't looked into this at all, so it's just a guess.