### 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)

Dictionary color_map;
	for (const Point2i &region : disabled_branch_regions) {
		if (p_line >= region.x && p_line <= region.y) {
			Dictionary highlighter_info;
			highlighter_info["color"] = disabled_branch_color;
			color_map[0] = highlighter_info;
			return color_map;
		}
	}
	return CodeHighlighter::_get_line_syntax_highlighting_impl(p_line);
}
void GDShaderSyntaxHighlighter::add_disabled_branch_region(const Point2i &p_region) {
	ERR_FAIL_COND(p_region.x < 0);
	ERR_FAIL_COND(p_region.y < 0);
	for (int i = 0; i < disabled_branch_regions.size(); i++) {
		ERR_FAIL_COND_MSG(disabled_branch_regions[i].x == p_region.x, "Branch region with a start line '" + itos(p_region.x) + "' already exists.");
	}
	Point2i disabled_branch_region;
	disabled_branch_region.x = p_region.x;
	disabled_branch_region.y = p_region.y;
	disabled_branch_regions.push_back(disabled_branch_region);
	clear_highlighting_cache();
}
void GDShaderSyntaxHighlighter::clear_disabled_branch_regions() {