"Uncaught TypeError: Cannot read property 'dom' of null" when saving

I've followed the Manage Child Resources tutorial from the RTFM, when I get to the very end and add a new resources under the parent, hit Done nothing happens. If I view the error consol I can see the following error appear each time I try and click Done:



I'm pretty sure I've followed the tutorial exactly. This is on MODX Cloud, MODX Revo version 2.2.4 with MIGX 2.3.0 [ed. note: AdamWintle last edited this post 11 years, 7 months ago.]