[processing] reset variables list in field calculator on layer change (…

…fix #15633)

python/plugins/processing/algs/qgis/ui/FieldsCalculatorDialog.py

@@ -105,7 +105,6 @@ def manageGui(self):
         self.mOutputFieldTypeComboBox.blockSignals(False)
         self.builder.loadRecent('fieldcalc')
 
-        self.initContext()
         self.updateLayer(self.cmbInputLayer.currentLayer())
 
     def initContext(self):
@@ -117,6 +116,8 @@ def initContext(self):
 
     def updateLayer(self, layer):
         self.layer = layer
+
+        self.initContext()
         self.builder.setLayer(self.layer)
         self.builder.loadFieldNames()
         self.populateFields()

src/gui/qgsexpressionbuilderwidget.cpp

@@ -516,8 +516,7 @@ void QgsExpressionBuilderWidget::setExpressionText( const QString &expression )
 void QgsExpressionBuilderWidget::setExpressionContext( const QgsExpressionContext &context )
 {
   mExpressionContext = context;
-
-  loadExpressionContext();
+  updateFunctionTree();
 }
 
 void QgsExpressionBuilderWidget::on_txtExpressionString_textChanged()