### Godot version

v4.2.dev.custom_build [eca6f0eb5]

### System information

Windows 10, Forward+, NVIDIA GeForce GTX 1050 Ti (31.0.15.3623)

### Issue description

When I edit my .gdshaderinc (a shader include file that I use for glsl shaders) file outside of Godot; and I go back to the Godot editor; it asks me if I want to `resave`, `reload` or `cancel`.
- `Resave` will revert the changes I made outside of Godot.
- `Reload` will use the changes I made outside of Godot.
- `Cancel` will do neither.

So I choose `reload` because I want to use the changes made outside the Godot editor.

However, every time I do, I get the following error message
```
editor\plugins\text_shader_editor.cpp:181 - Condition "shader.is_null()" is true.
```

### Steps to reproduce

- Make a .gdshaderinc file with for example some constants
- Make a .glsl file including the .gdshaderinc file
- Open the project with the Godot editor
- Edit and save the .gdshaderinc file outside of Godot
- Give focus to the Godot editor and choose `resave`
- Observe error

### Minimal reproduction project

This project contains a glsl_shader.glsl including a glsl_include.gdshaderinc file.
[MRP-shader-include-error.zip](https://github.com/godotengine/godot/files/12490157/MRP-shader-include-error.zip)

return shader_editor->get_text_editor()->get_saved_version() != shader_editor->get_text_editor()->get_version();
}
void TextShaderEditor::tag_saved_version() {
	shader_editor->get_text_editor()->tag_saved_version();
}
void TextShaderEditor::apply_shaders() {
	String editor_code = shader_editor->get_text_editor()->get_text();
	if (shader.is_valid()) {
		String shader_code = shader->get_code();
		if (shader_code != editor_code || dependencies_version != shader_editor->get_dependencies_version()) {
			shader_editor->set_block_shader_changed(true);
			shader->set_code(editor_code);
			shader_editor->set_block_shader_changed(false);
			shader->set_edited(true);
		}
	}
	if (shader_inc.is_valid()) {
		String shader_inc_code = shader_inc->get_code();
		if (shader_inc_code != editor_code || dependencies_version != shader_editor->get_dependencies_version()) {
			shader_editor->set_block_shader_changed(true);
			shader_inc->set_code(editor_code);
			shader_editor->set_block_shader_changed(false);
			shader_inc->set_edited(true);
		}
	}
	dependencies_version = shader_editor->get_dependencies_version();
}
void TextShaderEditor::_text_edit_gui_input(const Ref<InputEvent> &ev) {