StateError class Null safety
The operation was not allowed by the current state of the object.
Should be used when this particular object is currently in a state which doesn't support the requested operation, but other similar objects might, or the object itself can later change its state to one which supports the operation.
Example: Asking for
list.first on a currently empty list.
If the operation is never supported by this object or class,
consider using UnsupportedError instead.
This is a generic error used for a variety of different erroneous actions. The message should be descriptive.