Can't the parameter type 'Stream<MyUser?>?' be assigned with the argument type 'Stream<MyUser?>'?

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Yes, parameter type 'Stream<myuser?>?' can be assigned with the argument type 'Stream<myuser?>' as they are compatible with each other in terms of null safety. The question mark at the end of the parameter type 'Stream<myuser?>?' indicates that the value of the stream can be null, while the absence of the question mark in the argument type 'Stream<myuser?>' indicates that the value of the stream cannot be null. This means that the argument type is a subtype of the parameter type, and therefore, it can be assigned to the parameter type.