Prefab overrides crash unity with "Child already has a owner, it must be removed first."

[jtsmith1287]

I tried to repro this the best I could before filing this issue.

If you create a UI prefab that has a flexlayout on the root canvas. Have another UI prefab in a different scene, and add the first prefab. Make some changes on the parent, then attempt to override the child prefab with the change.

[gilzoide]

Hey @jtsmith1287, thanks for the report. We had this crash before. It seems I haven't tested all posibilities and it still ocurrs in some cases 🫠
If you could post a more detailed step-by-step for reproducing the crash, or maybe post a video showing exactly how you reproduce the crash, it would help to streamline the debug process. Also, if you have a stack trace showing the C# methods from FlexLayout called, it would also help.

[jtsmith1287]

I created a new project to try to reproduce this in isolation and I was not able to. I updated my unity to the most recent 2022.3 version, which is 2022.3.60f1. I was quite a few patches behind. That didn't resolve it.

In my main project I have UIs created independently. I have camera space canvases, and all my "parent" UI elements have a canvas. Basically each panel that contains a similar grouping of elements will have a canvas. Anyway, on the prefab the parent object doesn't have a flexlayout component. The prefab is saved with an empty rect transform as the root object. If I click and drag the prefab to be nested in a larger grouping of UI objects I found it didn't align correctly, obviously. I added flexlayout component to the empty parent object, which then aligned the content as expected. Then when I click Override on the new root prefab instance unity crashes and I get the "child already has owner" message.

I'm not sure what the intended workflow is. I assumed I could make UI prefabs and then add them wherever needed in the inspector, but that seems problematic. If I use a normal unity Layout group as a bridge between flexlayout objects I can get around the issue. Basically an existing group of flexlayouts can't merge with another one if the intention is then set those instant overrides, which makes building compound UIs difficult.

[jtsmith1287]

Maybe this is just forcing me to have good prefab hygiene. If I make sure to always edit the prefab directly, as in, within the contextual or isolation view, I'm able to safely add flexlayout components and return to the parent prefab. At least for existing objects. I still need to test adding an entirely new object with a flexlayout component to an existing one.

[gilzoide]

Hey @jtsmith1287 , thanks for the extra info.

I'm not sure what the intended workflow is. I assumed I could make UI prefabs and then add them wherever needed in the inspector, but that seems problematic.

The intended way to work with the package is exactly what you assumed: it should just work in all scenarios without problems. Without prefabs, with prefabs, nested prefabs, prefab overrides, shouldn't matter. If Unity is crashing, we surely have a bug.

What bugs me out is that you are getting a crash instead of just an error log. As of version 1.0.4 the C++ code should be catching the exception and returning it to C#, no crashes involved 🤔

I'll try reproducing and fixing this in the weekend.

[jtsmith1287]

Let me know if you need any more info from me. I wish I had more details, but without a log and not being able to repro in a separate project I can share I don't have much more. I will clarify that I had a container with a flex layout with children that did not have flexlayouts. I would then drag the prefab into the children that did not have the flexlayout, and would add the flexlayout component after. So effectively taking 2 flexlayout hierarchies and bridging them with the new component on the empty gameobject. At that point when I would click the apply overrides is when unity would crash. Hopefully that additional information helps track down the bug.

[gilzoide]

Hey @jtsmith1287, just here to say I was able to reproduce the bug by applying an override with a "bridge" between FlexLayout hierarchies to the source prefab asset, reproduces 100% of the time. Well, actually if I apply the bridging FlexLayout component without saving the prefab that's being edited, it doesn't happen. I have to save the parent prefab, then apply the new component to the source prefab.

In my case, it didn't crash. Say, which version of Flex UI are you using? From version 1.0.4 and onwards (we are already in 1.2.2) you should get a simple error message instead of crashing, just like in my screenshot.