When you have Custom Connection and use SelectField, Connection screen becomes unusable (see #17064) 

We should detect the situation and throw an exception when Provider Info is initialized.
airflow/providers_manager.py
"""Returns information about available providers."""
        self.initialize_providers_manager()
        return self._provider_dict
    @property
    def hooks(self) -> Dict[str, HookInfo]:
        """Returns dictionary of connection_type-to-hook mapping"""
        self.initialize_providers_manager()
        return self._hooks_dict
    @property
    def extra_links_class_names(self):
        """Returns set of extra link class names."""
        self.initialize_providers_manager()
        return sorted(self._extra_link_class_name_set)
    @property
    def connection_form_widgets(self) -> Dict[str, ConnectionFormWidgetInfo]:
        """Returns widgets for connection forms."""
        self.initialize_providers_manager()
        return self._connection_form_widgets
    @property
    def field_behaviours(self) -> Dict[str, Dict]:
        """Returns dictionary with field behaviours for connection types."""
        self.initialize_providers_manager()
        return self._field_behaviours