### 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)

shader_editor->get_text_editor()->paste();
		} break;
		case EDIT_SELECT_ALL: {
			shader_editor->get_text_editor()->select_all();
		} break;
		case EDIT_MOVE_LINE_UP: {
			shader_editor->move_lines_up();
		} break;
		case EDIT_MOVE_LINE_DOWN: {
			shader_editor->move_lines_down();
		} break;
		case EDIT_INDENT: {
			if (shader.is_null() && shader_inc.is_null()) {
				return;
			}
			shader_editor->get_text_editor()->indent_lines();
		} break;
		case EDIT_UNINDENT: {
			if (shader.is_null() && shader_inc.is_null()) {
				return;
			}
			shader_editor->get_text_editor()->unindent_lines();
		} break;
		case EDIT_DELETE_LINE: {
			shader_editor->delete_lines();
		} break;
		case EDIT_DUPLICATE_SELECTION: {
			shader_editor->duplicate_selection();
		} break;
		case EDIT_TOGGLE_WORD_WRAP: {