Cause
This error occurs if your application code assigns to a read-only property outside the owning class or one of its subclasses.
Properties can be defined as being read-only. The value of a read-only property is accessed by any method in any class, but it is assigned a new value only by methods within the class in which the property is defined, or one of its subclasses.
Action
Change your application code to remove the invalid assignment.
If you did not intend the property to be read-only, modify it by selecting the property in the Class Browser and then selecting the Change command from the Properties menu.