private static void checkNotNull(String fieldName, Object val, String fieldToSet) {
        if (val == null)
            throw new SchemaBuilderException("Invalid SchemaBuilder call: " + fieldName + " must be specified to set " + fieldToSet);
    }
Return a concrete instance of the {@link Schema} specified by this builder
@return the {@link Schema}
connect/api/src/main/java/org/apache/kafka/connect/data/SchemaBuilder.java