### 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)

disabled_branch_regions.clear();
	clear_highlighting_cache();
}
void GDShaderSyntaxHighlighter::set_disabled_branch_color(const Color &p_color) {
	disabled_branch_color = p_color;
	clear_highlighting_cache();
}
static bool saved_warnings_enabled = false;
static bool saved_treat_warning_as_errors = false;
static HashMap<ShaderWarning::Code, bool> saved_warnings;
static uint32_t saved_warning_flags = 0U;
void ShaderTextEditor::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_THEME_CHANGED: {
			if (is_visible_in_tree()) {
				_load_theme_settings();
				if (warnings.size() > 0 && last_compile_result == OK) {
					warnings_panel->clear();
					_update_warning_panel();
				}
			}
		} break;
	}
}
Ref<Shader> ShaderTextEditor::get_edited_shader() const {