### Godot version

3.5.1mono

### System information

windows10

### Issue description

Error message
 scene/main/node.cpp:1362 - Index p_index = 0 is out of bounds (data.children.size() = 0).
 scene/main/node.cpp:397 - Parameter "p_child" is null.
 scene/gui/tab_container.cpp:593 - Index p_current = 0 is out of bounds (get_tab_count() = 0).
See "Steps to reproduce for details"

### Steps to reproduce

Set an external text editor
Create a script
Drop the script to the built-in text editor like this image:
![image](https://user-images.githubusercontent.com/119792950/209504041-039f2620-5e42-4c0f-8c41-cd24486fb3d2.png)
And got error message:
 scene/main/node.cpp:1362 - Index p_index = 0 is out of bounds (data.children.size() = 0).
 scene/main/node.cpp:397 - Parameter "p_child" is null.
 scene/gui/tab_container.cpp:593 - Index p_current = 0 is out of bounds (get_tab_count() = 0).

### Minimal reproduction project

An empty project
switch (p_option) {
		case THEME_IMPORT: {
			file_dialog->set_file_mode(EditorFileDialog::FILE_MODE_OPEN_FILE);
			file_dialog->set_access(EditorFileDialog::ACCESS_FILESYSTEM);
			file_dialog_option = THEME_IMPORT;
			file_dialog->clear_filters();
			file_dialog->add_filter("*.tet");
			file_dialog->popup_file_dialog();
			file_dialog->set_title(TTR("Import Theme"));
		} break;
		case THEME_RELOAD: {
			EditorSettings::get_singleton()->load_text_editor_theme();
		} break;
		case THEME_SAVE: {
			if (EditorSettings::get_singleton()->is_default_text_editor_theme()) {
				ScriptEditor::_show_save_theme_as_dialog();
			} else if (!EditorSettings::get_singleton()->save_text_editor_theme()) {
				EditorNode::get_singleton()->show_warning(TTR("Error while saving theme"), TTR("Error saving"));
			}
		} break;
		case THEME_SAVE_AS: {
			ScriptEditor::_show_save_theme_as_dialog();
		} break;
	}
}
void ScriptEditor::_show_save_theme_as_dialog() {