Fix uninitialized AbstractScreen instance pointer.

...yeah yeah, one day I'm gonna zero out the kernel's BSS segment. Soon..

Kernel/WindowComposer.cpp

@@ -13,6 +13,7 @@ void WindowComposer_main()
     FrameBuffer::initialize();
     EventLoop::initialize();
     WindowManager::initialize();
+    AbstractScreen::initialize();
 
     auto info = current->get_display_info();
 

Widgets/AbstractScreen.cpp

@@ -6,6 +6,11 @@
 
 static AbstractScreen* s_the;
 
+void AbstractScreen::initialize()
+{
+    s_the = nullptr;
+}
+
 AbstractScreen& AbstractScreen::the()
 {
     ASSERT(s_the);

Widgets/AbstractScreen.h

@@ -16,6 +16,8 @@ public:
     Size size() const { return { width(), height() }; }
     Rect rect() const { return { 0, 0, width(), height() }; }
 
+    static void initialize();
+
 protected:
     AbstractScreen(unsigned width, unsigned height);