Properties: clear text ctrl from unit binder before it is destroyed

The lifetime of this text ctrl is managed by the property grid, so
we need to catch when it is hidden before destruction in order to avoid stale pointers being used to handle unit change events.

Credit to @dsa-t for the fix

common/widgets/unit_binder.cpp

@@ -642,5 +642,13 @@ void PROPERTY_EDITOR_UNIT_BINDER::SetControl( wxWindow* aControl )
                               nullptr, this );
         m_valueCtrl->Connect( wxEVT_LEFT_UP, wxMouseEventHandler( PROPERTY_EDITOR_UNIT_BINDER::onClick ),
                               nullptr, this );
+        m_valueCtrl->Bind( wxEVT_SET_FOCUS, &PROPERTY_EDITOR_UNIT_BINDER::onSetFocus, this );
+
+        m_valueCtrl->Bind( wxEVT_SHOW,
+                           [&]( wxShowEvent& e )
+                           {
+                               if( !e.IsShown() )
+                                   SetControl( nullptr );
+                           } );
     }
 }