HackStudio: Fix "Add new Editor" action

Author: https://github.com/itamar8910
Commit: dac7db830a
Pull-request: https://github.com/SerenityOS/serenity/pull/3470
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/devsh0

DevTools/HackStudio/Editor.cpp

@@ -48,6 +48,7 @@ namespace HackStudio {
 
 Editor::Editor()
 {
+    set_document(CodeDocument::create());
     m_documentation_tooltip_window = GUI::Window::construct();
     m_documentation_tooltip_window->set_rect(0, 0, 500, 400);
     m_documentation_tooltip_window->set_window_type(GUI::WindowType::Tooltip);