### 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)

_load_theme_settings();
	get_text_editor()->set_text(p_code);
	get_text_editor()->clear_undo_history();
	get_text_editor()->call_deferred(SNAME("set_h_scroll"), 0);
	get_text_editor()->call_deferred(SNAME("set_v_scroll"), 0);
	get_text_editor()->tag_saved_version();
	_validate_script();
	_line_col_changed();
}
void ShaderTextEditor::reload_text() {
	ERR_FAIL_COND(shader.is_null());
	CodeEdit *te = get_text_editor();
	int column = te->get_caret_column();
	int row = te->get_caret_line();
	int h = te->get_h_scroll();
	int v = te->get_v_scroll();
	te->set_text(shader->get_code());
	te->set_caret_line(row);
	te->set_caret_column(column);
	te->set_h_scroll(h);
	te->set_v_scroll(v);
	te->tag_saved_version();
	update_line_and_column();
}
void ShaderTextEditor::set_warnings_panel(RichTextLabel *p_warnings_panel) {
	warnings_panel = p_warnings_panel;
}
void ShaderTextEditor::_load_theme_settings() {