just catch all errors in json decode

app.py

@@ -74,9 +74,9 @@ def main(form):
   result['spatial_params']=spatparams
   result['distortion_params']=distparams
   result['points_params']=ptsparams
-  if 'detections' in form and isinstance(form['detections'], str):
+  try:
     result['detections']=json.loads(form['detections'])
-  else:
+  except:
     result['detections']=[]
   result['image_points']=[]