### 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)

CodeEdit *te = get_text_editor();
	Color updated_marked_line_color = EDITOR_GET("text_editor/theme/highlighting/mark_color");
	if (updated_marked_line_color != marked_line_color) {
		for (int i = 0; i < te->get_line_count(); i++) {
			if (te->get_line_background_color(i) == marked_line_color) {
				te->set_line_background_color(i, updated_marked_line_color);
			}
		}
		marked_line_color = updated_marked_line_color;
	}
	syntax_highlighter->set_number_color(EDITOR_GET("text_editor/theme/highlighting/number_color"));
	syntax_highlighter->set_symbol_color(EDITOR_GET("text_editor/theme/highlighting/symbol_color"));
	syntax_highlighter->set_function_color(EDITOR_GET("text_editor/theme/highlighting/function_color"));
	syntax_highlighter->set_member_variable_color(EDITOR_GET("text_editor/theme/highlighting/member_variable_color"));
	syntax_highlighter->clear_keyword_colors();
	const Color keyword_color = EDITOR_GET("text_editor/theme/highlighting/keyword_color");
	const Color control_flow_keyword_color = EDITOR_GET("text_editor/theme/highlighting/control_flow_keyword_color");
	List<String> keywords;
	ShaderLanguage::get_keyword_list(&keywords);
	for (const String &E : keywords) {
		if (ShaderLanguage::is_control_flow_keyword(E)) {
			syntax_highlighter->add_keyword_color(E, control_flow_keyword_color);
		} else {
			syntax_highlighter->add_keyword_color(E, keyword_color);
		}
	}
	List<String> pp_keywords;
	ShaderPreprocessor::get_keyword_list(&pp_keywords, false);
	for (const String &E : pp_keywords) {