### 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)

r_options->push_back(E);
		}
		return;
	}
	for (const ScriptLanguage::CodeCompletionOption &E : pp_defines) {
		r_options->push_back(E);
	}
	ShaderLanguage sl;
	String calltip;
	ShaderLanguage::ShaderCompileInfo comp_info;
	comp_info.global_shader_uniform_type_func = _get_global_shader_uniform_type;
	if (shader.is_null()) {
		comp_info.is_include = true;
		sl.complete(code, comp_info, r_options, calltip);
		get_text_editor()->set_code_hint(calltip);
		return;
	}
	_check_shader_mode();
	comp_info.functions = ShaderTypes::get_singleton()->get_functions(RenderingServer::ShaderMode(shader->get_mode()));
	comp_info.render_modes = ShaderTypes::get_singleton()->get_modes(RenderingServer::ShaderMode(shader->get_mode()));
	comp_info.shader_types = ShaderTypes::get_singleton()->get_types();
	sl.complete(code, comp_info, r_options, calltip);
	get_text_editor()->set_code_hint(calltip);
}
void ShaderTextEditor::_validate_script() {
	emit_signal(SNAME("script_changed")); 
	String code;
	if (shader.is_valid()) {
		_check_shader_mode();
		code = shader->get_code();