Add error message on empty public resources

This should provide more understandable behavior in the case described in
#5077

RELNOTES: none
PiperOrigin-RevId: 193968203

src/tools/android/java/com/google/devtools/build/android/PlaceholderIdFieldInitializerBuilder.java

@@ -337,13 +337,17 @@ private Map<ResourceType, Integer> assignTypeIdsForPublic() {
                 "Cannot force ATTR to have type code other than 0x%02x (got 0x%02x from %s)",
                 ATTR_TYPE_ID, reservedTypeSlot, previousResource));
       }
-      allocatedTypeIds.put(currentType, reservedTypeSlot);
-      ResourceType alreadyAssigned = assignedIds.put(reservedTypeSlot, currentType);
-      if (alreadyAssigned != null) {
-        logger.warning(
-            String.format(
-                "Multiple type names declared for public type identifier 0x%x (%s vs %s)",
-                reservedTypeSlot, alreadyAssigned, currentType));
+      if (reservedTypeSlot == null) {
+        logger.warning(String.format("Invalid public resource of type %s - ignoring", currentType));
+      } else {
+        allocatedTypeIds.put(currentType, reservedTypeSlot);
+        ResourceType alreadyAssigned = assignedIds.put(reservedTypeSlot, currentType);
+        if (alreadyAssigned != null) {
+          logger.warning(
+              String.format(
+                  "Multiple type names declared for public type identifier 0x%x (%s vs %s)",
+                  reservedTypeSlot, alreadyAssigned, currentType));
+        }
       }
     }
     return allocatedTypeIds;