Merge pull request #1454 from SamueleFurnari/fix_required_field_error_message

Initialise changedValue with defaultValue,

Author: joaojmendes

src/controls/dynamicForm/dynamicField/DynamicField.tsx

@@ -32,7 +32,7 @@ export class DynamicField extends React.Component<IDynamicFieldProps, IDynamicFi
       spfxContext: { pageContext: this.props.context.pageContext }
     });
     this.state = {
-      changedValue: props.fieldType === 'Thumbnail' ? props.fieldDefaultValue : null
+      changedValue: props.fieldDefaultValue
     };
   }