Bug 609674 - Don't remove pop3 activities in Activity manager that are not shown. r=mkmelin

mail/components/activity/content/activity.xml

@@ -198,6 +198,13 @@
         ]]>
         </getter>
       </property>
+      <property name="isGroup">
+        <getter>
+        <![CDATA[
+          return false;
+        ]]>
+        </getter>
+      </property>
       <property name="activity">
         <getter>
           <![CDATA[

mail/components/activity/modules/pop3Download.js

@@ -120,8 +120,10 @@ let pop3DownloadModule =
       // for this folder also didn't download any messages, remove the
       // prev event from the activity manager.
       let prevItem = this._prevActivityForFolder[aFolder.URI];
-      if (prevItem != undefined && !prevItem.numMsgsDownloaded)
-        this.activityMgr.removeActivity(prevItem.eventID);
+      if (prevItem != undefined && !prevItem.numMsgsDownloaded) {
+        if (this.activityMgr.containsActivity(prevItem.eventID))
+          this.activityMgr.removeActivity(prevItem.eventID);
+      }
     }
   },
   init: function() {