If layer changes in stats summary dock, reset field to empty

This prevents a stats gatherer task immediately firing off
on a random field. Otherwise if the current layer is removed
from the project, a new (random) layer will be selected
in the stats dock and a task will fire off gathering (unwanted)
stats for it. If multiple layers are removed this can trigger
many gatherers, all of which are unwanted.

src/app/qgsstatisticalsummarydockwidget.cpp

@@ -154,7 +154,7 @@ void QgsStatisticalSummaryDockWidget::copyStatistics()
 
 void QgsStatisticalSummaryDockWidget::refreshStatistics()
 {
-  if ( !mLayer || ( mFieldExpressionWidget->isExpression() && !mFieldExpressionWidget->isValidExpression() ) )
+  if ( !mLayer || mFieldExpressionWidget->currentField().isEmpty() || ( mFieldExpressionWidget->isExpression() && !mFieldExpressionWidget->isValidExpression() ) )
   {
     mStatisticsTable->setRowCount( 0 );
     return;
@@ -346,13 +346,15 @@ void QgsStatisticalSummaryDockWidget::layerChanged( QgsMapLayer *layer )
 
   mLayer = newLayer;
 
+  // clear expression, so that we don't force an unwanted recalculation
+  mFieldExpressionWidget->setExpression( QString() );
+  mFieldExpressionWidget->setLayer( mLayer );
+
   if ( mLayer )
   {
     connect( mLayer, &QgsVectorLayer::selectionChanged, this, &QgsStatisticalSummaryDockWidget::layerSelectionChanged );
   }
 
-  mFieldExpressionWidget->setLayer( mLayer );
-
   if ( mGatherer )
   {
     mGatherer->cancel();