Fix NPE that occurs on invalid user input in editfields (#1077)

Signed-off-by: Natalia Kislicyn <natalia.kislicyn@bosch.io>

hawkbit-ui/src/main/java/org/eclipse/hawkbit/ui/common/data/proxies/ProxyIdentifiableEntity.java

@@ -88,6 +88,6 @@ public abstract class ProxyIdentifiableEntity implements Serializable {
 
     @Override
     public String toString() {
-        return id.toString();
+        return String.valueOf(id);
     }
 }