Hi Jun,
As stated in
Chartboost's Mediation Help Site, the adapter automatically sets all named locations to Default if you do not set one in the AdMob Ad Review Center. The warning simply states that.
As for your second concern, the contructors for initializing ads requires a Context object from an Activity instance, as stated in the warning.
However, can you send us a sample code snippet on how you are implementing your ads so we can investigate further?
Regards,
Joshua Lagonera
Mobile Ads SDK Team