Return a BrowserView instance by the :value of its given :attribute,
        and None if no match is found.

def get_instance(attr, value):
        """
        Return a BrowserView instance by the :value of its given :attribute,
        and None if no match is found.
        """
        for i in list(BrowserView.instances.values()):
            try:
                if getattr(i, attr) == value:
                    return i
            except AttributeError:
                break

        return None