Prevent possible NPE in error message. Closes #173.

Author: joehni

xstream/src/java/com/thoughtworks/xstream/core/DefaultConverterLookup.java

@@ -1,6 +1,6 @@
 /*
  * Copyright (C) 2004, 2005, 2006 Joe Walnes.
- * Copyright (C) 2006, 2007, 2008, 2009, 2011, 2013, 2014, 2015, 2016, 2017 XStream Committers.
+ * Copyright (C) 2006, 2007, 2008, 2009, 2011, 2013, 2014, 2015, 2016, 2017, 2019 XStream Committers.
  * All rights reserved.
  *
  * The software in this package is published under the terms of the BSD
@@ -76,7 +76,7 @@ public Converter lookupConverterForType(final Class<?> type) {
         final ConversionException exception = new ConversionException(errors.isEmpty()
             ? "No converter specified"
             : "No converter available");
-        exception.add("type", type.getName());
+        exception.add("type", type != null ? type.getName() : "null");
         for (final Map.Entry<String, String> entry : errors.entrySet()) {
             exception.add("converter", entry.getKey());
             exception.add("message", entry.getValue());