### 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)

} else {
		code = shader_inc->get_code();
	}
	ShaderPreprocessor preprocessor;
	String code_pp;
	String error_pp;
	List<ShaderPreprocessor::FilePosition> err_positions;
	List<ShaderPreprocessor::Region> regions;
	String filename;
	if (shader.is_valid()) {
		filename = shader->get_path();
	} else if (shader_inc.is_valid()) {
		filename = shader_inc->get_path();
	}
	last_compile_result = preprocessor.preprocess(code, filename, code_pp, &error_pp, &err_positions, &regions);
	for (int i = 0; i < get_text_editor()->get_line_count(); i++) {
		get_text_editor()->set_line_background_color(i, Color(0, 0, 0, 0));
	}
	syntax_highlighter->clear_disabled_branch_regions();
	for (const ShaderPreprocessor::Region &region : regions) {
		if (!region.enabled) {
			if (filename != region.file) {
				continue;
			}
			syntax_highlighter->add_disabled_branch_region(Point2i(region.from_line, region.to_line));
		}
	}
	set_error("");
	set_error_count(0);
	if (last_compile_result != OK) {