[gnome-documents] lokview: Avoid backtrace when LOK isn't available

[Debarshi Ray <debarshir src gnome org>]

commit 0e5f1d814db33afb7368b1aa356646dabbd7a4a0
Author: Bastien Nocera <hadess hadess net>
Date:   Mon Dec 7 12:10:34 2015 +0100

    lokview: Avoid backtrace when LOK isn't available
    
    The error was already showing up, but a backtrace as well.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=753686

 src/lokview.js |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/src/lokview.js b/src/lokview.js
index 68bcde0..e19268f 100644
--- a/src/lokview.js
+++ b/src/lokview.js
@@ -144,6 +144,8 @@ const LOKView = new Lang.Class({
     _onLoadStarted: function(manager, doc) {
         if (doc.viewType != Documents.ViewType.LOK)
             return;
+        if (!isAvailable())
+            return;
         this._doc = doc;
         this.view.open_document(doc.uri, "{}", null, Lang.bind(this, this.open_document_cb));
         this._progressBar.show();