[fix] fastapi: init inner_exception properly

It looks to me that if the app does not yield anything for the response
you might encour into an attribute error as the attribut won't be defined yet.

In general, the init method sounds like a better place for shared attributes.

Fixes OCA#487

fastapi/fastapi_dispatcher.py

@@ -13,6 +13,11 @@
 class FastApiDispatcher(Dispatcher):
     routing_type = "fastapi"
 
+    def __init__(self, request):
+        super().__init__(request)
+        # Store exception to later raise it in the dispatch method if needed
+        self.inner_exception = None
+
     @classmethod
     def is_compatible_with(cls, request):
         return True
@@ -47,7 +52,6 @@ def handle_error(self, exc):
     def _make_response(self, status_mapping, headers_tuple, content):
         self.status = status_mapping[:3]
         self.headers = dict(headers_tuple)
-        self.inner_exception = None
         # in case of exception, the method asgi_done_callback of the
         # ASGIResponder will trigger an "a2wsgi.error" event with the exception
         # instance stored in a tuple with the type of the exception and the traceback.