Fix NullPointerException in Fingerprinter

getStacktrace in an exception can be overridden and return null.
handle this case gracefully

Author: jpbempel

dd-java-agent/agent-debugger/src/main/java/com/datadog/debugger/exception/Fingerprinter.java

@@ -32,12 +32,14 @@ public static String fingerprint(Throwable t, ClassNameFilter classNameFiltering
     String typeName = clazz.getTypeName();
     digest.update(typeName.getBytes());
     StackTraceElement[] stackTrace = t.getStackTrace();
-    for (StackTraceElement stackTraceElement : stackTrace) {
-      String className = stackTraceElement.getClassName();
-      if (classNameFiltering.isExcluded(className)) {
-        continue;
+    if (stackTrace != null) {
+      for (StackTraceElement stackTraceElement : stackTrace) {
+        String className = stackTraceElement.getClassName();
+        if (classNameFiltering.isExcluded(className)) {
+          continue;
+        }
+        digest.update(stackTraceElement.toString().getBytes());
       }
-      digest.update(stackTraceElement.toString().getBytes());
     }
     return bytesToHex(digest.digest());
   }

dd-java-agent/agent-debugger/src/test/java/com/datadog/debugger/exception/FingerprinterTest.java

@@ -69,6 +69,20 @@ void emptyStacktrace() {
         Fingerprinter.fingerprint(new EmptyException("test"), classNameFiltering));
   }
 
+  @Test
+  void nullStacktrace() {
+    assertEquals(
+        "35ae5d9aa4d7179a7d36838ca6266ea459a7cbb6ebc92afc24098bc85cad586",
+        Fingerprinter.fingerprint(
+            new RuntimeException("test") {
+              @Override
+              public StackTraceElement[] getStackTrace() {
+                return null;
+              }
+            },
+            classNameFiltering));
+  }
+
   static class EmptyException extends Exception {
     public EmptyException(String message) {
       super(message, null, false, false);