Fix life cycle of ResourceImporterTexture not tracked properly

[RandomShaper]

Fixes comment in #79936.

This class is instantiated as part of the generation of docs, and later the object is destroyed. By not nullifying the singleton, the next instantiation (the primary one done by the editor), the false assumption that it's already instantiated is made and a dangling pointer to the destroyed singleton is used.

[YuriSizov]

Not sure it closes the linked issue (we don't really have much info on the cause of that), but it should fix the problem that @aaronfranke is having.

[YuriSizov]

Makes sense, we can merge it on its own merit. Would be great to get a confirmation that it's the exact cause of crash for Aaron.

[YuriSizov]

Thanks!

[aaronfranke]

I tested with commit a0f5724 and I am getting a different error message in the crash:

================================================================
handle_crash: Program crashed with signal 11
Engine version: Godot Engine v4.2.dev.custom_build (a0f572494e6602e0927bc3de61e9093c37da60f0)
Dumping the backtrace. Please include this when reporting the bug to the project developer.
[1] 1   libsystem_platform.dylib            0x0000000184362a24 _sigtramp + 56
[2] 2   ???                                 0xffff80018425f104 0x0 + 18446603342733242628
[3] std::__1::recursive_mutex::lock()
[4] ResourceImporterTexture::update_imports() (in godot.macos.editor.arm64) + 96
[5] EditorNode::_notification(int) (in godot.macos.editor.arm64) + 2764
[6] EditorPluginList::~EditorPluginList()
[7] Object::notification(int, bool) (in godot.macos.editor.arm64) + 32
[8] SceneTree::_process_group(SceneTree::ProcessGroup*, bool) (in godot.macos.editor.arm64) + 280
[9] SceneTree::_process(bool) (in godot.macos.editor.arm64) + 676
[10] SceneTree::process(double) (in godot.macos.editor.arm64) + 240
[11] Main::iteration() (in godot.macos.editor.arm64) + 952
[12] OS_MacOS::run() (in godot.macos.editor.arm64) + 116
[13] main (in godot.macos.editor.arm64) + 304
[14] 14  dyld                                0x0000000183fdbf28 start + 2236
-- END OF BACKTRACE --
================================================================

However, it seems to not be crashing on a new empty project. I am using this project to test the crash.

Also, like before, the crash seems to be intermittent. If I clear the cache it may work, or not. It's really weird.