improved logging in case a device mapping points to an object of unknown type or a null object

Change-Id: I54accd8b386af796b12bcfe95f0cbdba55a6b265

java/com.sap.sailing.gwt.ui/src/main/java/com/sap/sailing/gwt/ui/server/SailingServiceImpl.java

@@ -447,6 +447,7 @@ import com.sap.sse.common.TypeBasedServiceFinderFactory;
 import com.sap.sse.common.Util;
 import com.sap.sse.common.Util.Pair;
 import com.sap.sse.common.Util.Triple;
+import com.sap.sse.common.WithID;
 import com.sap.sse.common.impl.MillisecondsTimePoint;
 import com.sap.sse.common.impl.TimeRangeImpl;
 import com.sap.sse.common.mail.MailException;
@@ -5356,12 +5357,15 @@ public class SailingServiceImpl extends ProxiedRemoteServiceServlet implements S
         Date to = mapping.getTimeRange().to() == null || mapping.getTimeRange().to().asMillis() == Long.MAX_VALUE ?
                 null : mapping.getTimeRange().to().asDate();
         MappableToDevice item = null;
-        if (mapping.getMappedTo() instanceof Competitor) {
+        final WithID mappedTo = mapping.getMappedTo();
+        if (mappedTo == null) {
+            throw new RuntimeException("Device mapping not mapped to any object");
+        } else if (mappedTo instanceof Competitor) {
             item = baseDomainFactory.convertToCompetitorDTO((Competitor) mapping.getMappedTo());
-        } else if (mapping.getMappedTo() instanceof Mark) {
+        } else if (mappedTo instanceof Mark) {
             item = convertToMarkDTO((Mark) mapping.getMappedTo(), null);
         } else {
-            throw new RuntimeException("Can only handle Competitor or Mark as mapped item type");
+            throw new RuntimeException("Can only handle Competitor or Mark as mapped item type, but not "+mappedTo.getClass().getName());
         }
         //Only deal with UUIDs - otherwise we would have to pass Serializable to browser context - which
         //has a large performance implact for GWT.