ScrubForwarder pushes and pops itself on the window handler stack...

... Fixing crashes on close of project window seen in some Linux builds of
2.3.3.  The bug began at commit a3b434be1c

src/tracks/ui/ScrubUI.cpp

@@ -20,6 +20,7 @@
 
 #include <wx/dcclient.h>
 #include <wx/event.h>
+#include <wx/windowptr.h>
 
 ///////////////////////////////////////////////////////////////////////////////
 // class ScrubbingOverlay is responsible for drawing the speed numbers
@@ -218,9 +219,22 @@ struct ScrubForwarder
     : public wxEvtHandler
     , public ClientData::Base
 {
-   ScrubForwarder( AudacityProject &project ) : mProject{ project } {}
+   ScrubForwarder( AudacityProject &project )
+      : mProject{ project }
+   {
+      mWindow = &ProjectWindow::Get( project );
+      if ( mWindow )
+         mWindow->PushEventHandler( this );
+   }
+
+   ~ScrubForwarder()
+   {
+      if ( mWindow )
+         mWindow->PopEventHandler();
+   }
 
    AudacityProject &mProject;
+   wxWindowPtr<wxWindow> mWindow;
 
    void OnMouse(wxMouseEvent &event);
    DECLARE_EVENT_TABLE()
@@ -269,9 +283,6 @@ END_EVENT_TABLE()
 
 static const AudacityProject::AttachedObjects::RegisteredFactory sForwarderKey{
    []( AudacityProject &parent ){
-      auto result = std::make_shared< ScrubForwarder >( parent );
-      auto &window = ProjectWindow::Get( parent );
-      window.PushEventHandler( result.get() );
-      return result;
+      return std::make_shared< ScrubForwarder >( parent );
    }
 };

src/tracks/ui/Scrubbing.cpp

@@ -241,9 +241,6 @@ Scrubber::~Scrubber()
    if (mpThread)
       mpThread->Delete();
 #endif
-
-   if ( mWindow )
-      mWindow->PopEventHandler();
 }
 
 static const auto HasWaveDataPred =