fix flask undefined client (elastic#1099)

elasticapm/contrib/flask/__init__.py

@@ -88,17 +88,9 @@ def __init__(self, app=None, client=None, client_cls=Client, logging=False, **de
         self.app = app
         self.logging = logging
         self.client = client or get_client()
+        self.client_cls = client_cls
 
         if app:
-            if not self.client:
-                config = app.config.get("ELASTIC_APM", {})
-
-                if "framework_name" not in defaults:
-                    defaults["framework_name"] = "flask"
-                    defaults["framework_version"] = getattr(flask, "__version__", "<0.7")
-
-                self.client = client_cls(config, **defaults)
-
             self.init_app(app, **defaults)
 
     def handle_exception(self, *args, **kwargs):
@@ -123,6 +115,14 @@ def handle_exception(self, *args, **kwargs):
 
     def init_app(self, app, **defaults):
         self.app = app
+        if not self.client:
+            config = self.app.config.get("ELASTIC_APM", {})
+
+            if "framework_name" not in defaults:
+                defaults["framework_name"] = "flask"
+                defaults["framework_version"] = getattr(flask, "__version__", "<0.7")
+
+            self.client = self.client_cls(config, **defaults)
 
         # 0 is a valid log level (NOTSET), so we need to check explicitly for it
         if self.logging or self.logging is logging.NOTSET: