### 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)

RS::GlobalShaderParameterType gvt = RS::get_singleton()->global_shader_parameter_get_type(p_variable);
	return (ShaderLanguage::DataType)RS::global_shader_uniform_type_get_shader_datatype(gvt);
}
static String complete_from_path;
static void _complete_include_paths_search(EditorFileSystemDirectory *p_efsd, List<ScriptLanguage::CodeCompletionOption> *r_options) {
	if (!p_efsd) {
		return;
	}
	for (int i = 0; i < p_efsd->get_file_count(); i++) {
		if (p_efsd->get_file_type(i) == SNAME("ShaderInclude")) {
			String path = p_efsd->get_file_path(i);
			if (path.begins_with(complete_from_path)) {
				path = path.replace_first(complete_from_path, "");
			}
			r_options->push_back(ScriptLanguage::CodeCompletionOption(path, ScriptLanguage::CODE_COMPLETION_KIND_FILE_PATH));
		}
	}
	for (int j = 0; j < p_efsd->get_subdir_count(); j++) {
		_complete_include_paths_search(p_efsd->get_subdir(j), r_options);
	}
}
static void _complete_include_paths(List<ScriptLanguage::CodeCompletionOption> *r_options) {