Fix exception mapping with exit codes

- Migitates changed behaviour in boot how exceptions are
  handled thus caused trouble with exit code mappings
  on a shell side.
- Fixes #961

spring-shell-core/src/main/java/org/springframework/shell/command/CommandRegistration.java

@@ -1080,7 +1080,7 @@ public interface CommandRegistration {
 		@Override
 		public ExitCodeSpec map(Class<? extends Throwable> e, int code) {
 			Function<Throwable, Integer> f = t -> {
-				if (ObjectUtils.nullSafeEquals(t.getClass(), e)) {
+				if (t != null && ObjectUtils.nullSafeEquals(t.getClass(), e)) {
 					return code;
 				}
 				return 0;