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
"""Lazy initialization of provider data."""
        
        
        
        
        
        if self._initialized:
            return
        
        
        
        
        self._discover_all_airflow_builtin_providers_from_local_sources()
        self._discover_all_providers_from_packages()
        self._discover_hooks()
        self._provider_dict = OrderedDict(sorted(self._provider_dict.items()))
        self._hooks_dict = OrderedDict(sorted(self._hooks_dict.items()))
        self._connection_form_widgets = OrderedDict(sorted(self._connection_form_widgets.items()))
        self._field_behaviours = OrderedDict(sorted(self._field_behaviours.items()))
        self._discover_extra_links()
        self._initialized = True
    def _discover_all_providers_from_packages(self) -> None: