When deleting a map canvas, first clear all the pointers to that

canvas from child map tools

Otherwise we risk the map tools trying to do things with the
canvas within their cleanup code, and at that stage the canvas
is already partially destroyed.

On qt 5 builds this leads to undefined behavior, and on qt 6
builds it triggers a newly introduced assert designed to
catch these kinds of bugs.

It's possible that we should just delete the map tool children
upfront here instead, but that's a little more risky.

src/gui/qgsmapcanvas.cpp

@@ -253,8 +253,20 @@ QgsMapCanvas::~QgsMapCanvas()
   if ( mMapTool )
   {
     mMapTool->deactivate();
+    disconnect( mMapTool, &QObject::destroyed, this, &QgsMapCanvas::mapToolDestroyed );
     mMapTool = nullptr;
   }
+
+  // we also clear the canvas pointer for all child map tools. We're now in a partially destroyed state and it's
+  // no longer safe for map tools to try to cleanup things in the canvas during their destruction (such as removing
+  // associated canvas items)
+  // NOTE -- it may be better to just delete the map tool children here upfront?
+  const QList< QgsMapTool * > tools = findChildren< QgsMapTool *>();
+  for ( QgsMapTool *tool : tools )
+  {
+    tool->mCanvas = nullptr;
+  }
+
   mLastNonZoomMapTool = nullptr;
 
   cancelJobs();

src/gui/qgsmaptool.h

@@ -353,6 +353,7 @@ class GUI_EXPORT QgsMapTool : public QObject
     //! The translated name of the map tool
     QString mToolName;
 
+    friend class QgsMapCanvas;
     friend class TestQgsMapToolEdit;
 
 };