Unable to get property _destroyed of undefined or null reference
If you are looking for a quick (dirty) fix:
Get the original mxui.js for your modeler version
Copy it to your theme folder and change the reference to the mxui.js file in your theme's index.html to the one in your theme folder
In the mxui.js file, search for "return hash[node.getAttribute("widgetId")];" and replace this with "return hash[node.getAttribute("widgetid")];"
Save & restart project
The thing is mxui.js does a getAttribute on widgetId with uppercase 'i'. However, the attributes in the DOM are all lowercase and altough I think getAttribute is supposed to be case-insensitive, it only works when you use widgetid with lowercase 'i' in the getAttribute.
At least this worked for me as a temporary hotfix untill a cleaner solution or fix from Mendix comes available.
I have seen this same issue today when testing one of our widgets. Reproduced this in 6.10.10.
I have pinpointed the part of code that causes this problem in our source. Seems like Edge is doing some weird voodoo here, as it works in a normal fashion in other browsers.
I have submitted a ticket with Support and will see if R&D can fix this issue.
Edit (June 7): I have been informed that they have a fix for this, currently testing it. If all goes well, this will be fixed in 6.10.13 and 7.16 (upcoming release)