Noticed the same error in couple of other scenarios in Firefox, they all might be coming from the same source, so listing here for now:

1. When inserting code block
2. While inserting Gallery
3. After clicking on a slug
4. Also the link inspector looks quite broken, opening up in random positions and throwing the same error.

In T175943#3609981, @Deskana wrote:

This is not in production yet, fortunately.

Now it is :)

I do see this in production, but not locally or in beta. Maybe we've fixed it accidentally somewhere else.

@Ryasmeen I can't reproduce this in production in Firefox.

1. Go to a random article, e.g. https://en.wikipedia.org/wiki/Bo_people_(China)
2. Click "Insert", then "Media".
3. Pick a random image.
4. Click "Use this image".
5. Click "Insert".

The image is inserted just fine.

Oh, also, in production that node is null error happens any time you hit enter, not just linked to the image-add.

Hmm, does not work for me on that page, getting same error. Mine is Firefox 55.0.3.

I just made this edit using Firefox 55.0.3: https://en.wikipedia.org/w/index.php?title=User:Deskana_(WMF)/sandbox&diff=801256835&oldid=800072580

@Ryasmeen: Could you check and let me know whether this happens to you on the beta cluster currently? I'm trying to work out whether I'm just having problems reproducing it, or if it's fixed, and Dan's trouble reproducing it in production makes everything ambiguous there.

I can reproduce it in both beta and production, and I figured out why. If I press enter at any point after VisualEditor loading and trying to insert the image, then I get a node is null error in my browser console, and when I try to insert an image I encounter this issue. If I never press enter, then it will insert the image just fine.

Okay, bisect indicates that both the node is null error and the dialog issue were introduced by 37d1512, @dchan's TransactionProcessor patch.

I think this is essentially the same as T175852 (which https://gerrit.wikimedia.org/r/378378/ fixes)

Change 378378 had a related patch set uploaded (by Divec; owner: Divec):
[VisualEditor/VisualEditor@master] Setup slugs in TreeModifier

https://gerrit.wikimedia.org/r/378378

Change 378378 merged by jenkins-bot:
[VisualEditor/VisualEditor@master] Setup slugs in TreeModifier

https://gerrit.wikimedia.org/r/378378

Change 379328 had a related patch set uploaded (by Jforrester; owner: Divec):
[VisualEditor/VisualEditor@REL1_30] Setup slugs in TreeModifier

https://gerrit.wikimedia.org/r/379328

Change 379328 merged by jenkins-bot:
[VisualEditor/VisualEditor@REL1_30] Setup slugs in TreeModifier

https://gerrit.wikimedia.org/r/379328

Change 379557 had a related patch set uploaded (by DLynch; owner: DLynch):
[mediawiki/extensions/VisualEditor@master] Update VE core submodule to master (bd5da1725)

https://gerrit.wikimedia.org/r/379557

Change 379557 merged by jenkins-bot:
[mediawiki/extensions/VisualEditor@master] Update VE core submodule to master (0ec5fe55f)

https://gerrit.wikimedia.org/r/379557