### 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)

case BOOKMARK_GOTO_NEXT: {
			shader_editor->goto_next_bookmark();
		} break;
		case BOOKMARK_GOTO_PREV: {
			shader_editor->goto_prev_bookmark();
		} break;
		case BOOKMARK_REMOVE_ALL: {
			shader_editor->remove_all_bookmarks();
		} break;
		case HELP_DOCS: {
			OS::get_singleton()->shell_open(vformat("%s/tutorials/shaders/shader_reference/index.html", VERSION_DOCS_URL));
		} break;
	}
	if (p_option != SEARCH_FIND && p_option != SEARCH_REPLACE && p_option != SEARCH_GOTO_LINE) {
		shader_editor->get_text_editor()->call_deferred(SNAME("grab_focus"));
	}
}
void TextShaderEditor::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_ENTER_TREE:
		case NOTIFICATION_THEME_CHANGED: {
			PopupMenu *popup = help_menu->get_popup();
			popup->set_item_icon(popup->get_item_index(HELP_DOCS), get_editor_theme_icon(SNAME("ExternalLink")));
		} break;
		case NOTIFICATION_APPLICATION_FOCUS_IN: {
			_check_for_external_edit();
		} break;
	}
}
void TextShaderEditor::_editor_settings_changed() {