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

Author: jvalkeal

spring-shell-autoconfigure/src/main/java/org/springframework/shell/boot/ExitCodeAutoConfiguration.java

@@ -59,7 +59,11 @@ static class ShellExitCodeExceptionMapper implements ExitCodeExceptionMapper {
 
 		@Override
 		public int getExitCode(Throwable exception) {
-			if (exception.getCause() instanceof CommandExecution.CommandParserExceptionsException) {
+			// for e vs. its cause, see gh-961
+			if (exception instanceof CommandExecution.CommandParserExceptionsException) {
+				return 2;
+			}
+			else if (exception.getCause() instanceof CommandExecution.CommandParserExceptionsException) {
 				return 2;
 			}
 			// only map parsing error so that other mappers can do their job
@@ -83,7 +87,11 @@ public void reset(List<Function<Throwable, Integer>> functions) {
 		public int getExitCode(Throwable exception) {
 			int exitCode = 0;
 			for (Function<Throwable, Integer> function : functions) {
-				Integer code = function.apply(exception.getCause());
+				Throwable cause = exception.getCause();
+				if (cause == null) {
+					cause = exception;
+				}
+				Integer code = function.apply(cause);
 				if (code != null) {
 					if (code > 0 && code > exitCode || code < 0 && code < exitCode) {
 						exitCode = code;

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

@@ -1080,7 +1080,7 @@ static class DefaultExitCodeSpec implements ExitCodeSpec {
 		@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;