[gnome-builder] html-preview: fix invalid use of view

[Christian Hergert <chergert src gnome org>]

commit fa0b5ddbbbac83ab94b0be10f3f624c8e9f6c3b7
Author: Christian Hergert <chergert redhat com>
Date:   Thu Sep 21 01:36:02 2017 -0700

    html-preview: fix invalid use of view

 src/plugins/html-preview/html_preview.py |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/plugins/html-preview/html_preview.py b/src/plugins/html-preview/html_preview.py
index 59f5c51..e748741 100644
--- a/src/plugins/html-preview/html_preview.py
+++ b/src/plugins/html-preview/html_preview.py
@@ -256,7 +256,8 @@ class HtmlPreviewAddin(GObject.Object, Ide.EditorViewAddin):
     def preview_activated(self, *args):
         global can_preview_rst
 
-        if self.view is None:
+        view = self.view
+        if view is None:
             return
 
         if self.lang_id == 'rst':