make sure we do not crash on best activity during compound validation

fix #207

ippisite/ippidb/models.py

@@ -862,7 +862,11 @@ class Compound(AutoFillableModel):
         best_pXC50_activity = self.best_activity
         if best_pXC50_activity is None:
             return None
-        return self.compoundactivityresult_set.filter(activity=best_pXC50_activity)[0]
+        best_cars = self.compoundactivityresult_set.filter(activity=best_pXC50_activity)
+        if len(best_cars) > 0:
+            return best_cars[0]
+        else:
+            return None
 
     @property
     def bioch_tests_count(self):