Fix NPE from StructuralSpecification

Change-Id: I67fbfa5b926b0c8a58efad0a7f565b7ecf21b7df
diff --git a/src/main/java/com/android/tools/r8/graph/DexMethodHandle.java b/src/main/java/com/android/tools/r8/graph/DexMethodHandle.java
index 227270d..63c3261 100644
--- a/src/main/java/com/android/tools/r8/graph/DexMethodHandle.java
+++ b/src/main/java/com/android/tools/r8/graph/DexMethodHandle.java
@@ -323,7 +323,7 @@
         .withConditionalItem(DexMethodHandle::isFieldHandle, DexMethodHandle::asField)
         .withConditionalItem(DexMethodHandle::isMethodHandle, DexMethodHandle::asMethod)
         .withBool(m -> m.isInterface)
-        .withItem(m -> m.rewrittenTarget);
+        .withNullableItem(m -> m.rewrittenTarget);
   }
 
   public Handle toAsmHandle() {