Fix NPE when trying to view file diff for closed change

Support for closed changes is still bad (e.g. the Code-Review buttons
should not be displayed at all), but at least the app is now no longer
crashing for closed changes.

Change-Id: Ide729d1b7a6fdd5c7b78e9678d5dbeada953828f
Signed-off-by: Edwin Kempin <ekempin@google.com>
diff --git a/app/src/main/java/com/google/reviewit/widget/SelectCodeReviewView.java b/app/src/main/java/com/google/reviewit/widget/SelectCodeReviewView.java
index 1c9bd1e..5ee5ce2 100644
--- a/app/src/main/java/com/google/reviewit/widget/SelectCodeReviewView.java
+++ b/app/src/main/java/com/google/reviewit/widget/SelectCodeReviewView.java
@@ -212,14 +212,18 @@
   public void setChange(Change change) {
     this.change = change;
 
-    Collection<Integer> voteableValues = Collections2.transform(
-        change.info.permittedLabels.get("Code-Review"),
-        new Function<String, Integer>() {
-      @Override
-      public Integer apply(String value) {
-        return Integer.parseInt(value.trim());
-      }
-    });
+    Collection<String> permittedCodeReviewVotes =
+        change.info.permittedLabels.get("Code-Review");
+
+    Collection<Integer> voteableValues = permittedCodeReviewVotes != null
+        ? Collections2.transform(permittedCodeReviewVotes,
+          new Function<String, Integer>() {
+            @Override
+            public Integer apply(String value) {
+              return Integer.parseInt(value.trim());
+            }
+          })
+        : new ArrayList<Integer>();
     for (VotingButton b : votingButtons) {
       if (!voteableValues.contains(b.getVote())) {
         b.setDisabled(true);