Fix crash in attribute form after adding field

Fixes #18507

Author: nyalldawson

src/gui/qgsattributeform.cpp

@@ -1108,6 +1108,7 @@ void QgsAttributeForm::init()
   vl->addWidget( container );
 
   mFormEditorWidgets.clear();
+  mFormWidgets.clear();
 
   // a bar to warn the user with non-blocking messages
   setContentsMargins( 0, 0, 0, 0 );