### Godot version

v4.0 rc6

### System information

Windows 10

### Issue description

When using a ViewportTexture from a SubViewport on a TextureRect, it throws errors after running the scene once and then switching scenes in the editor.
![Pasted image 20230227212626](https://user-images.githubusercontent.com/44676282/221680047-984fa03e-33d1-4856-9a7f-1700b28423cf.png)


### Steps to reproduce

Run the canvas-scene once.
Switch to another scene in the editor.
Switch back to the canvas-scene

### Minimal reproduction project

[ViewportTextureError.zip](https://github.com/godotengine/godot/files/10844099/ViewportTextureError.zip)

ERR_READ_THREAD_GUARD_V(0);
	return positional_shadow_atlas_size;
}
void Viewport::set_positional_shadow_atlas_16_bits(bool p_16_bits) {
	ERR_MAIN_THREAD_GUARD;
	if (positional_shadow_atlas_16_bits == p_16_bits) {
		return;
	}
	positional_shadow_atlas_16_bits = p_16_bits;
	RS::get_singleton()->viewport_set_positional_shadow_atlas_size(viewport, positional_shadow_atlas_size, positional_shadow_atlas_16_bits);
}
bool Viewport::get_positional_shadow_atlas_16_bits() const {
	ERR_READ_THREAD_GUARD_V(false);
	return positional_shadow_atlas_16_bits;
}
void Viewport::set_positional_shadow_atlas_quadrant_subdiv(int p_quadrant, PositionalShadowAtlasQuadrantSubdiv p_subdiv) {
	ERR_MAIN_THREAD_GUARD;
	ERR_FAIL_INDEX(p_quadrant, 4);
	ERR_FAIL_INDEX(p_subdiv, SHADOW_ATLAS_QUADRANT_SUBDIV_MAX);
	if (positional_shadow_atlas_quadrant_subdiv[p_quadrant] == p_subdiv) {
		return;
	}
	positional_shadow_atlas_quadrant_subdiv[p_quadrant] = p_subdiv;
	static const int subdiv[SHADOW_ATLAS_QUADRANT_SUBDIV_MAX] = { 0, 1, 4, 16, 64, 256, 1024 };
	RS::get_singleton()->viewport_set_positional_shadow_atlas_quadrant_subdivision(viewport, p_quadrant, subdiv[p_subdiv]);
}
Viewport::PositionalShadowAtlasQuadrantSubdiv Viewport::get_positional_shadow_atlas_quadrant_subdiv(int p_quadrant) const {