ChangeNotes: Use checkNotNull rather than checkArgument to check for null

This prevents false-positive warnings from FindBugs on subsequent
dereferences of the variables that have been null-checked.

Change-Id: Ifacd9f6be29f572d2689ad670e0c9507174901e8
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeNotes.java b/gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeNotes.java
index 16f4d4b..24256bf 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeNotes.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/notedb/ChangeNotes.java
@@ -163,7 +163,7 @@
     public ChangeNotes create(ReviewDb db, Project.NameKey project,
         Change.Id changeId) throws OrmException {
       Change change = unwrap(db).changes().get(changeId);
-      checkArgument(change != null,
+      checkNotNull(change,
           "change %s not found in ReviewDb", changeId);
       checkArgument(change.getProject().equals(project),
           "passed project %s when creating ChangeNotes for %s, but actual"
@@ -196,7 +196,7 @@
       checkState(!args.migration.readChanges(), "do not call"
           + " createFromIdOnlyWhenNoteDbDisabled when NoteDb is enabled");
       Change change = unwrap(db).changes().get(changeId);
-      checkArgument(change != null,
+      checkNotNull(change,
           "change %s not found in ReviewDb", changeId);
       return new ChangeNotes(args, change.getProject(), change).load();
     }