Fix NullPointerException caused by 0acba61522 calling getMacAddress when DiscoveryActivity might not have had deviceTarget set

Discovered via Bangle.js App Store ANR reporting

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/activities/discovery/DiscoveryActivityV2.java

@@ -806,6 +806,7 @@ public class DiscoveryActivityV2 extends AbstractGBActivity implements AdapterVi
 
     @Override
     public String getMacAddress() {
+        if (deviceTarget==null) return null;
         return deviceTarget.getDevice().getAddress();
     }