canvas::NasalWidget: ensure nasal part is destroyed

simgear/canvas/layout/NasalWidget.cxx

@@ -33,6 +33,12 @@ namespace canvas
 
   }
 
+  //----------------------------------------------------------------------------
+  NasalWidget::~NasalWidget()
+  {
+    onRemove();
+  }
+
   //----------------------------------------------------------------------------
   void NasalWidget::invalidate()
   {

simgear/canvas/layout/NasalWidget.hxx

@@ -49,6 +49,8 @@ namespace canvas
        */
       NasalWidget(naRef impl);
 
+      ~NasalWidget();
+
       virtual void invalidate();
       virtual void setGeometry(const SGRecti& geom);
       virtual void onRemove();