Fix button constant name.

Change-Id: I7358e67849c50e619203c91c9029c2010c0d54b3
diff --git a/src/com/android/mail/ui/MailboxSelectionActivity.java b/src/com/android/mail/ui/MailboxSelectionActivity.java
index 55c267c..f588344 100644
--- a/src/com/android/mail/ui/MailboxSelectionActivity.java
+++ b/src/com/android/mail/ui/MailboxSelectionActivity.java
@@ -94,7 +94,7 @@
                 actionBar.setIcon(R.mipmap.ic_launcher_shortcut_folder);
             }
         }
-        ((Button) findViewById(R.id.cancel)).setOnClickListener(this);
+        ((Button) findViewById(R.id.first_button)).setOnClickListener(this);
 
         // Initially, assume that the main view is invisible.  It will be made visible,
         // if we display the account list