I'm getting javascript error "popupBar is null on popupBar.onmousedown = this.initDND ;" when migrating from EPP 5.1 to 5.2.

Issue

I use the groovy template files inherited from EPP 5.1. After I migrate to EPP 5.2 I get javascript errors:

    popupBar is null on
    popupBar.onmousedown = this.initDND ;