### 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)

if (shader.is_valid()) {
		_reload_shader_from_disk();
	} else if (shader_inc.is_valid()) {
		_reload_shader_include_from_disk();
	}
}
void TextShaderEditor::edit(const Ref<Shader> &p_shader) {
	if (p_shader.is_null() || !p_shader->is_text_shader()) {
		return;
	}
	if (shader == p_shader) {
		return;
	}
	shader = p_shader;
	shader_inc = Ref<ShaderInclude>();
	shader_editor->set_edited_shader(shader);
}
void TextShaderEditor::edit(const Ref<ShaderInclude> &p_shader_inc) {
	if (p_shader_inc.is_null()) {
		return;
	}
	if (shader_inc == p_shader_inc) {
		return;
	}
	shader_inc = p_shader_inc;
	shader = Ref<Shader>();
	shader_editor->set_edited_shader_include(p_shader_inc);
}
void TextShaderEditor::save_external_data(const String &p_str) {