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
"""
    Manages all provider packages. This is a Singleton class. The first time it is
    instantiated, it discovers all available providers in installed packages and
    local source folders (if airflow is run from sources).
    """
    _instance = None
    resource_version = "0"
    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance
    def __init__(self):
        
        self._provider_dict: Dict[str, ProviderInfo] = {}
        
        self._hooks_dict: Dict[str, HookInfo] = {}
        
        self._connection_form_widgets: Dict[str, ConnectionFormWidgetInfo] = {}
        
        self._field_behaviours: Dict[str, Dict] = {}
        self._extra_link_class_name_set: Set[str] = set()
        self._provider_schema_validator = _create_provider_info_schema_validator()
        self._customized_form_fields_schema_validator = (
            _create_customized_form_field_behaviours_schema_validator()
        )
        self._initialized = False
    def initialize_providers_manager(self):