[processing] fix error when adding refactor fields alg in empty model

(fixes #17421)

Author: nirvn

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

@@ -477,7 +477,8 @@ def createWidget(self):
     def postInitialize(self, wrappers):
         for wrapper in wrappers:
             if wrapper.param.name() == self.param.parentLayerParameter():
-                self.setLayer(wrapper.value())
+                if wrapper.value():
+                    self.setLayer(wrapper.value())
                 wrapper.widgetValueHasChanged.connect(self.parentLayerChanged)
                 break
 

python/plugins/processing/gui/wrappers.py

@@ -999,7 +999,10 @@ def validator(v):
                 else:
                     return os.path.exists(v)
 
-            return self.comboValue(validator, combobox=self.combo)
+            if self.combo.currentText():
+                return self.comboValue(validator, combobox=self.combo)
+            else:
+                return None
 
 
 class StringWidgetWrapper(WidgetWrapper):