Weird variable not used warning since compiler SDK 4.2.1

My code compiles with SDK 4.1.7 with no errors and no warnings.

With SDK 4.2.1 the compiler is complaining on a variable in a class that is not used. But it is.

Can anyone help here, what's going wrong?

The warning:

ForumsladerApp.mc:29: Member variable '_deviceManager' is not used.

The code:

Variable is declared here and used here.