Attempt to fix NPE in Enqueuer#traverseHierarchy.

Change-Id: I73206f3bab088e1ff2349790674698b7b29604e0
diff --git a/src/main/java/com/android/tools/r8/shaking/Enqueuer.java b/src/main/java/com/android/tools/r8/shaking/Enqueuer.java
index 04f10d9..f744579 100644
--- a/src/main/java/com/android/tools/r8/shaking/Enqueuer.java
+++ b/src/main/java/com/android/tools/r8/shaking/Enqueuer.java
@@ -2076,7 +2076,7 @@
     // TODO(b/70160030): Revise this to support tree shaking library methods on non-escaping types.
     DexProgramClass holder = getProgramClassOrNull(method.holder);
     if (holder == null) {
-      // TODO: clean this.
+      // TODO(b/139464956): clean this.
       // Ensure that the full proto of the targeted method is referenced.
       recordTypeReference(method.proto.returnType);
       for (DexType type : method.proto.parameters.values) {
@@ -2494,6 +2494,9 @@
     while (!worklist.isEmpty()) {
       DexType type = worklist.pop();
       DexClass definition = appView.definitionFor(type);
+      if (definition == null) {
+        continue;
+      }
       if (definition.isProgramClass()) {
         // TODO(b/120884788): This should assert not possible once fixed.
         continue;