fix mypy errors

mypy_django_plugin/transformers/meta.py

@@ -1,4 +1,5 @@
 from django.core.exceptions import FieldDoesNotExist
+from mypy.nodes import TypeInfo
 from mypy.plugin import MethodContext
 from mypy.types import AnyType, Instance
 from mypy.types import Type as MypyType
@@ -9,11 +10,16 @@ from mypy_django_plugin.lib import fullnames, helpers
 
 
 def _get_field_instance(ctx: MethodContext, field_fullname: str) -> MypyType:
-    field_info = helpers.lookup_fully_qualified_typeinfo(ctx.api, field_fullname)
+    field_info = helpers.lookup_fully_qualified_typeinfo(helpers.get_typechecker_api(ctx),
+                                                         field_fullname)
+    assert isinstance(field_info, TypeInfo)
     return Instance(field_info, [AnyType(TypeOfAny.explicit), AnyType(TypeOfAny.explicit)])
 
 
 def return_proper_field_type_from_get_field(ctx: MethodContext, django_context: DjangoContext) -> MypyType:
+    # Options instance
+    assert isinstance(ctx.type, Instance)
+
     model_type = ctx.type.args[0]
     if not isinstance(model_type, Instance):
         return _get_field_instance(ctx, fullnames.FIELD_FULLNAME)