[processing] fixed field loading in field calculator

fixes #15767

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

Author: volaya

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

@@ -91,6 +91,7 @@ def manageGui(self):
         self.builder.loadRecent('fieldcalc')
 
         self.initContext()
+        self.updateLayer()
 
     def initContext(self):
         exp_context = self.builder.expressionContext()
@@ -171,7 +172,8 @@ def toggleNewGroup(self, toggled):
     def populateFields(self):
         if self.layer is None:
             return
-
+        
+        self.mExistingFieldComboBox.clear()
         fields = self.layer.fields()
         for f in fields:
             self.mExistingFieldComboBox.addItem(f.name())