Merge pull request #5901 from effective-webwork/3-6-prevent-npe-helper

[3.6] prevent npe helper

Kitodo/src/main/java/org/kitodo/production/helper/Helper.java

@@ -490,6 +490,7 @@ private static String appendUnusedInsertions(String message, String... insertion
         StringBuilder messageBuilder = new StringBuilder(message);
         for (String insertion : insertions) {
             String separator = ": ";
+            insertion = Objects.toString(insertion);
             if (!messageBuilder.toString().contains(insertion)) {
                 messageBuilder.append(separator).append(insertion);
             }