[gedit-code-assistance/protect] Added check for empty doc

[Jesse van den Kieboom <jessevdk src gnome org>]

commit f5974d72ea96d18d71c3b1d1b839730214a1cd12
Author: Jesse van den Kieboom <jessevdk gmail com>
Date:   Mon Jan 13 16:32:27 2014 +0100

    Added check for empty doc

 src/gca-backend.vala |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)
---
diff --git a/src/gca-backend.vala b/src/gca-backend.vala
index 524b07f..00fd1eb 100644
--- a/src/gca-backend.vala
+++ b/src/gca-backend.vala
@@ -145,6 +145,13 @@ class Backend : Object
        {
                var doc = v.document;
 
+               if (doc == null)
+               {
+                       // This happens when a document has been closed while we're
+                       // iterating over open views.
+                       return null;
+               }
+
                if (doc.is_modified)
                {
                        try