LibGUI: Turn GUI::Application::the() into a pointer

During app teardown, the Application object may be destroyed before
something else, and so having Application::the() return a reference was
obscuring the truth about its lifetime.

This patch makes the API more honest by returning a pointer. While
this makes call sites look a bit more sketchy, do note that the global
Application pointer only becomes null during app teardown.

Libraries/LibGUI/Action.cpp

@@ -157,14 +157,17 @@ Action::Action(const StringView& text, const Shortcut& shortcut, RefPtr<Gfx::Bit
         m_scope = ShortcutScope::WindowLocal;
     } else {
         m_scope = ShortcutScope::ApplicationGlobal;
-        Application::the().register_global_shortcut_action({}, *this);
+        if (auto* app = Application::the())
+            app->register_global_shortcut_action({}, *this);
     }
 }
 
 Action::~Action()
 {
-    if (m_shortcut.is_valid() && m_scope == ShortcutScope::ApplicationGlobal)
-        Application::the().unregister_global_shortcut_action({}, *this);
+    if (m_shortcut.is_valid() && m_scope == ShortcutScope::ApplicationGlobal) {
+        if (auto* app = Application::the())
+            app->unregister_global_shortcut_action({}, *this);
+    }
 }
 
 void Action::activate(Core::Object* activator)

Libraries/LibGUI/Application.cpp

@@ -39,18 +39,17 @@
 
 namespace GUI {
 
-static Application* s_the;
+static WeakPtr<Application> s_the;
 
-Application& Application::the()
+Application* Application::the()
 {
-    ASSERT(s_the);
-    return *s_the;
+    return s_the;
 }
 
 Application::Application(int argc, char** argv)
 {
     ASSERT(!s_the);
-    s_the = this;
+    s_the = make_weak_ptr();
     m_event_loop = make<Core::EventLoop>();
     WindowServerConnection::the();
     Clipboard::initialize({});
@@ -68,7 +67,7 @@ Application::Application(int argc, char** argv)
 
 Application::~Application()
 {
-    s_the = nullptr;
+    revoke_weak_ptrs();
 }
 
 int Application::exec()

Libraries/LibGUI/Application.h

@@ -40,7 +40,7 @@ class Application : public Core::Object {
     C_OBJECT(Application);
 
 public:
-    static Application& the();
+    static Application* the();
 
     ~Application();
 

Libraries/LibGUI/Widget.cpp

@@ -98,7 +98,7 @@ Widget::Widget()
     , m_background_role(Gfx::ColorRole::Window)
     , m_foreground_role(Gfx::ColorRole::WindowText)
     , m_font(Gfx::Font::default_font())
-    , m_palette(Application::the().palette().impl())
+    , m_palette(Application::the()->palette().impl())
 {
 }
 
@@ -245,7 +245,7 @@ void Widget::handle_paint_event(PaintEvent& event)
         painter.draw_rect(rect(), Color::Magenta);
     }
 
-    if (Application::the().focus_debugging_enabled()) {
+    if (Application::the()->focus_debugging_enabled()) {
         if (is_focused()) {
             Painter painter(*this);
             painter.draw_rect(rect(), Color::Cyan);
@@ -318,13 +318,13 @@ void Widget::handle_mousedoubleclick_event(MouseEvent& event)
 void Widget::handle_enter_event(Core::Event& event)
 {
     if (has_tooltip())
-        Application::the().show_tooltip(m_tooltip, screen_relative_rect().center().translated(0, height() / 2));
+        Application::the()->show_tooltip(m_tooltip, screen_relative_rect().center().translated(0, height() / 2));
     enter_event(event);
 }
 
 void Widget::handle_leave_event(Core::Event& event)
 {
-    Application::the().hide_tooltip();
+    Application::the()->hide_tooltip();
     leave_event(event);
 }
 

Libraries/LibGUI/Window.cpp

@@ -113,7 +113,7 @@ void Window::show()
     apply_icon();
 
     reified_windows->set(m_window_id, this);
-    Application::the().did_create_window({});
+    Application::the()->did_create_window({});
     update();
 }
 
@@ -158,7 +158,7 @@ void Window::hide()
         }
     }
     if (!app_has_visible_windows)
-        Application::the().did_delete_last_window({});
+        Application::the()->did_delete_last_window({});
 }
 
 void Window::set_title(const StringView& title)

Libraries/LibGUI/WindowServerConnection.cpp

@@ -61,7 +61,7 @@ static void set_system_theme_from_shbuf_id(int id)
     auto system_theme = SharedBuffer::create_from_shbuf_id(id);
     ASSERT(system_theme);
     Gfx::set_system_theme(*system_theme);
-    Application::the().set_system_palette(*system_theme);
+    Application::the()->set_system_palette(*system_theme);
 }
 
 void WindowServerConnection::handshake()
@@ -154,7 +154,7 @@ void WindowServerConnection::handle(const Messages::WindowClient::KeyDown& messa
     }
 
     if (!action) {
-        action = Application::the().action_for_key_event(*key_event);
+        action = Application::the()->action_for_key_event(*key_event);
 #ifdef KEYBOARD_SHORTCUTS_DEBUG
         dbg() << "  > Asked application, got action: " << action;
 #endif