### 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)

if (p_idx < 4) { 
		_menu_option(bookmarks_menu->get_item_id(p_idx));
	} else {
		shader_editor->goto_line(bookmarks_menu->get_item_metadata(p_idx));
	}
}
void TextShaderEditor::_make_context_menu(bool p_selection, Vector2 p_position) {
	context_menu->clear();
	if (p_selection) {
		context_menu->add_shortcut(ED_GET_SHORTCUT("ui_cut"), EDIT_CUT);
		context_menu->add_shortcut(ED_GET_SHORTCUT("ui_copy"), EDIT_COPY);
	}
	context_menu->add_shortcut(ED_GET_SHORTCUT("ui_paste"), EDIT_PASTE);
	context_menu->add_separator();
	context_menu->add_shortcut(ED_GET_SHORTCUT("ui_text_select_all"), EDIT_SELECT_ALL);
	context_menu->add_shortcut(ED_GET_SHORTCUT("ui_undo"), EDIT_UNDO);
	context_menu->add_shortcut(ED_GET_SHORTCUT("ui_redo"), EDIT_REDO);
	context_menu->add_separator();
	context_menu->add_shortcut(ED_GET_SHORTCUT("script_text_editor/indent"), EDIT_INDENT);
	context_menu->add_shortcut(ED_GET_SHORTCUT("script_text_editor/unindent"), EDIT_UNINDENT);
	context_menu->add_shortcut(ED_GET_SHORTCUT("script_text_editor/toggle_comment"), EDIT_TOGGLE_COMMENT);
	context_menu->add_shortcut(ED_GET_SHORTCUT("script_text_editor/toggle_bookmark"), BOOKMARK_TOGGLE);
	context_menu->set_position(get_screen_position() + p_position);
	context_menu->reset_size();
	context_menu->popup();
}
TextShaderEditor::TextShaderEditor() {