objc: Some methods did not check for null NSError** param.

[abarnert]

• Many of the methods in AEMApplication, AEMEvent, etc. follow
  the usual ObjC idiom of taking an optional NSError** error, but
  some of the methods in AEMApplication are missing checks for
  null, and therefore crash on *error=nil.

For example:

[AEMApplication findApplicationForCreator:kLSUnknownCreator
                                 bundleID:nil
                                     name:@"NonexistentApp"
                                    error:nil];