Merge pull request #2741 from theuni/fix-osx-dock

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

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];
 }
@@ -113,9 +114,12 @@ MacDockIconHandler *MacDockIconHandler::instance()
 }
 
 void MacDockIconHandler::handleDockIconClickEvent()
+{
+    if (this->mainWindow)
     {
         this->mainWindow->activateWindow();
         this->mainWindow->show();
+    }
 
     emit this->dockIconClicked();
 }