### 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)

int warning_count = 0;
	warnings_panel->push_table(2);
	for (int i = 0; i < warnings.size(); i++) {
		ShaderWarning &w = warnings[i];
		if (warning_count == 0) {
			if (saved_treat_warning_as_errors) {
				String error_text = "error(" + itos(w.get_line()) + "): " + w.get_message() + " " + TTR("Warnings should be fixed to prevent errors.");
				set_error_pos(w.get_line() - 1, 0);
				set_error(error_text);
				get_text_editor()->set_line_background_color(w.get_line() - 1, marked_line_color);
			}
		}
		warning_count++;
		int line = w.get_line();
		
		warnings_panel->push_cell();
		warnings_panel->push_color(warnings_panel->get_theme_color(SNAME("warning_color"), EditorStringName(Editor)));
		if (line != -1) {
			warnings_panel->push_meta(line - 1);
			warnings_panel->add_text(TTR("Line") + " " + itos(line));
			warnings_panel->add_text(" (" + w.get_name() + "):");
			warnings_panel->pop(); 
		} else {
			warnings_panel->add_text(w.get_name() + ":");
		}
		warnings_panel->pop(); 
		warnings_panel->pop();