### 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)

TextEdit::LineWrappingMode wrap = shader_editor->get_text_editor()->get_line_wrapping_mode();
			shader_editor->get_text_editor()->set_line_wrapping_mode(wrap == TextEdit::LINE_WRAPPING_BOUNDARY ? TextEdit::LINE_WRAPPING_NONE : TextEdit::LINE_WRAPPING_BOUNDARY);
		} break;
		case EDIT_TOGGLE_COMMENT: {
			if (shader.is_null() && shader_inc.is_null()) {
				return;
			}
			shader_editor->toggle_inline_comment("//");
		} break;
		case EDIT_COMPLETE: {
			shader_editor->get_text_editor()->request_code_completion();
		} break;
		case SEARCH_FIND: {
			shader_editor->get_find_replace_bar()->popup_search();
		} break;
		case SEARCH_FIND_NEXT: {
			shader_editor->get_find_replace_bar()->search_next();
		} break;
		case SEARCH_FIND_PREV: {
			shader_editor->get_find_replace_bar()->search_prev();
		} break;
		case SEARCH_REPLACE: {
			shader_editor->get_find_replace_bar()->popup_replace();
		} break;
		case SEARCH_GOTO_LINE: {
			goto_line_dialog->popup_find_line(shader_editor->get_text_editor());
		} break;
		case BOOKMARK_TOGGLE: {
			shader_editor->toggle_bookmark();
		} break;