[3d] fix crash when switching terrain generator (fixes #21538)

The problem was introduced in PR #8828 when fixing issue #20963

This fix immediately deletes terrain entity on generator change
and informs other code about that + identify map tool is now aware
of the fact that terrain entity may be temporarily null.

(cherry picked from commit 4a6151a)

src/3d/qgs3dmapscene.cpp

@@ -365,6 +365,8 @@ void Qgs3DMapScene::createTerrain()
 
     mTerrain->deleteLater();
     mTerrain = nullptr;
+
+    emit terrainEntityChanged();
   }
 
   if ( !mTerrainUpdateScheduled )

src/3d/qgs3dmapscene.h

@@ -59,7 +59,7 @@ class _3D_EXPORT Qgs3DMapScene : public Qt3DCore::QEntity
 
     //! Returns camera controller
     QgsCameraController *cameraController() { return mCameraController; }
-    //! Returns terrain entity
+    //! Returns terrain entity (may be temporarily null)
     QgsTerrainEntity *terrainEntity() { return mTerrain; }
 
     //! Resets camera view to show the whole scene (top view)

src/app/3d/qgs3dmaptoolidentify.cpp

@@ -77,16 +77,20 @@ void Qgs3DMapToolIdentify::mousePressEvent( QMouseEvent *event )
 
 void Qgs3DMapToolIdentify::activate()
 {
-  Qt3DRender::QObjectPicker *picker = mCanvas->scene()->terrainEntity()->terrainPicker();
-  connect( picker, &Qt3DRender::QObjectPicker::clicked, this, &Qgs3DMapToolIdentify::onTerrainPicked );
+  if ( QgsTerrainEntity *terrainEntity = mCanvas->scene()->terrainEntity() )
+  {
+    connect( terrainEntity->terrainPicker(), &Qt3DRender::QObjectPicker::clicked, this, &Qgs3DMapToolIdentify::onTerrainPicked );
+  }
 
   mCanvas->scene()->registerPickHandler( mPickHandler.get() );
 }
 
 void Qgs3DMapToolIdentify::deactivate()
 {
-  Qt3DRender::QObjectPicker *picker = mCanvas->scene()->terrainEntity()->terrainPicker();
-  disconnect( picker, &Qt3DRender::QObjectPicker::clicked, this, &Qgs3DMapToolIdentify::onTerrainPicked );
+  if ( QgsTerrainEntity *terrainEntity = mCanvas->scene()->terrainEntity() )
+  {
+    disconnect( terrainEntity->terrainPicker(), &Qt3DRender::QObjectPicker::clicked, this, &Qgs3DMapToolIdentify::onTerrainPicked );
+  }
 
   mCanvas->scene()->unregisterPickHandler( mPickHandler.get() );
 }
@@ -137,6 +141,8 @@ void Qgs3DMapToolIdentify::onTerrainEntityChanged()
 {
   // no need to disconnect from the previous entity: it has been destroyed
   // start listening to the new terrain entity
-  Qt3DRender::QObjectPicker *picker = mCanvas->scene()->terrainEntity()->terrainPicker();
-  connect( picker, &Qt3DRender::QObjectPicker::clicked, this, &Qgs3DMapToolIdentify::onTerrainPicked );
+  if ( QgsTerrainEntity *terrainEntity = mCanvas->scene()->terrainEntity() )
+  {
+    connect( terrainEntity->terrainPicker(), &Qt3DRender::QObjectPicker::clicked, this, &Qgs3DMapToolIdentify::onTerrainPicked );
+  }
 }