Fix NPE in maindex tracing

Handle classes without definition.

Bug: 202173862
Change-Id: Id04483cbff63beda703d91be7f483effe32bfb7d
diff --git a/src/main/java/com/android/tools/r8/shaking/MainDexListBuilder.java b/src/main/java/com/android/tools/r8/shaking/MainDexListBuilder.java
index b648635..ffa265c 100644
--- a/src/main/java/com/android/tools/r8/shaking/MainDexListBuilder.java
+++ b/src/main/java/com/android/tools/r8/shaking/MainDexListBuilder.java
@@ -129,8 +129,9 @@
             DexType valueType = proto.returnType.toBaseType(appView.dexItemFactory());
             if (valueType.isClassType()) {
               assert !value;
+              DexClass valueTypeClass = appInfo().definitionFor(valueType);
               boolean notLibraryOrTakeBootClasspath =
-                  !appInfo().definitionFor(valueType).isLibraryClass()
+                  (valueTypeClass != null && !valueTypeClass.isLibraryClass())
                       || !appView.options().ignoreBootClasspathEnumsForMaindexTracing;
               value =
                   (isEnum(valueType) && notLibraryOrTakeBootClasspath)