Fixed color picker crashes the engine on Linux

[pwnSquirrel]

Operating system or device - Godot version:
Arch Linux 64 - Latest build, master branch

Issue description (what happened, and what was expected):
The fixed color picker (from pull request #7431) crashes the engine when being used:

ERROR: get_child: Index p_index out of size (data.children.size()).
At: scene/main/node.cpp:1521.

This also happens immediately when I try to open an old project. A new empty poject can be opened, but it crashes as soon as I try to use the color picker (in project settings, for example).

I reverted that commit, recompiled and it worked again. So it is definitely caused by it.

Steps to reproduce:
Create empty project. Open it. Go to project settings/Render. Click on the default background color to pick a new one.

Link to minimal example project (optional but very welcome):

[pwnSquirrel]

It seems to work now (no crash anymore). However, the color picker doesn't look like it should:

And the crosshairs don't move at all when I'm clicking.

[MarianoGnu]

Yep, i have been testing this and want to fix it, but for some reason drawing colored polygons (non textured) is not working at all so it's a different issue, not sure if i can fix it.

[nunodonato]

I can't reproduce this crash (Linux, Godot 3.0 master), but I opened a new issue for the color picker glitch #8372

[MarianoGnu]

I'm closing this issue since the glitch has moved to #8372