Fix an NPE in BatteryService

mCallback is possible to be null. Avoid registering null callbacks.

Bug: 175184519
Test: TH
Change-Id: Ic382d268f40a8ea31aaeedc7a85b6dc9b4e6c210
Merged-In: Ic382d268f40a8ea31aaeedc7a85b6dc9b4e6c210
(cherry picked from commit 780ca87adc)

services/core/java/com/android/server/BatteryService.java

@@ -1505,6 +1505,8 @@ public final class BatteryService extends SystemService {
                             if (Objects.equals(newService, oldService)) return;
 
                             Slog.i(TAG, "health: new instance registered " + mInstanceName);
+                            // #init() may be called with null callback. Skip null callbacks.
+                            if (mCallback == null) return;
                             mCallback.onRegistration(oldService, newService, mInstanceName);
                         } catch (NoSuchElementException | RemoteException ex) {
                             Slog.e(TAG, "health: Cannot get instance '" + mInstanceName