constructor(initialState) {
        addReadProp(this, '_inner', {}, true);
        if (initialState && typeof initialState === 'object') {
            this.extendState(initialState);
        }
    }
@private
@class InnerState
@description
Implements support for private/inner state object inside the class,
which can be accessed by a derived class via hidden read-only property _inner.