Fix sendComplete UI updates on destroyed views

Fixes #2915
Closes #2924

// FREEBIE

src/org/thoughtcrime/securesms/ConversationActivity.java

@@ -974,7 +974,7 @@ public class ConversationActivity extends PassphraseRequiredActionBarActivity
     boolean refreshFragment = (threadId != this.threadId);
     this.threadId = threadId;
 
-    if (fragment == null) {
+    if (fragment == null || !fragment.isVisible() || isFinishing()) {
       return;
     }