### 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)

edit_menu->get_popup()->add_separator();
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("ui_cut"), EDIT_CUT);
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("ui_copy"), EDIT_COPY);
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("ui_paste"), EDIT_PASTE);
	edit_menu->get_popup()->add_separator();
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("ui_text_select_all"), EDIT_SELECT_ALL);
	edit_menu->get_popup()->add_separator();
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/move_up"), EDIT_MOVE_LINE_UP);
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/move_down"), EDIT_MOVE_LINE_DOWN);
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/indent"), EDIT_INDENT);
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/unindent"), EDIT_UNINDENT);
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/delete_line"), EDIT_DELETE_LINE);
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/toggle_comment"), EDIT_TOGGLE_COMMENT);
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/duplicate_selection"), EDIT_DUPLICATE_SELECTION);
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/toggle_word_wrap"), EDIT_TOGGLE_WORD_WRAP);
	edit_menu->get_popup()->add_separator();
	edit_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("ui_text_completion_query"), EDIT_COMPLETE);
	edit_menu->get_popup()->connect("id_pressed", callable_mp(this, &TextShaderEditor::_menu_option));
	search_menu = memnew(MenuButton);
	search_menu->set_shortcut_context(this);
	search_menu->set_text(TTR("Search"));
	search_menu->set_switch_on_hover(true);
	search_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/find"), SEARCH_FIND);
	search_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/find_next"), SEARCH_FIND_NEXT);
	search_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/find_previous"), SEARCH_FIND_PREV);
	search_menu->get_popup()->add_shortcut(ED_GET_SHORTCUT("script_text_editor/replace"), SEARCH_REPLACE);
	search_menu->get_popup()->connect("id_pressed", callable_mp(this, &TextShaderEditor::_menu_option));
	MenuButton *goto_menu = memnew(MenuButton);
	goto_menu->set_shortcut_context(this);
	goto_menu->set_text(TTR("Go To"));