osx: fix bitcoin-qt startup crash when clicking dock icon

Crash probably introduced by 4d17a1b0.
Inialize the window to NULL and verify it before use.

src/qt/macdockiconhandler.mm

@@ -51,6 +51,7 @@ MacDockIconHandler::MacDockIconHandler() : QObject()
     this->m_dockIconClickEventHandler = [[DockIconClickEventHandler alloc] initWithDockIconHandler:this];
     this->m_dummyWidget = new QWidget();
     this->m_dockMenu = new QMenu(this->m_dummyWidget);
+    this->setMainWindow(NULL);
 
     [pool release];
 }
@@ -114,8 +115,11 @@ MacDockIconHandler *MacDockIconHandler::instance()
 
 void MacDockIconHandler::handleDockIconClickEvent()
 {
+    if (this->mainWindow)
+    {
         this->mainWindow->activateWindow();
         this->mainWindow->show();
+    }
 
     emit this->dockIconClicked();
 }