[gnome-builder] symbol-tree: only update tree with new valid tree

[Christian Hergert <chergert src gnome org>]

commit f6e0f9d005b0894a043d027bce0b1fc94de18e1d
Author: Christian Hergert <chergert redhat com>
Date:   Mon Sep 11 14:00:05 2017 -0700

    symbol-tree: only update tree with new valid tree
    
    It can happen that we get a parse error because the user was typing and
    left the source in a non-compilable state.

 .../symbol-tree/gbp-symbol-layout-stack-addin.c    |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)
---
diff --git a/plugins/symbol-tree/gbp-symbol-layout-stack-addin.c 
b/plugins/symbol-tree/gbp-symbol-layout-stack-addin.c
index 6ddef56..1392da4 100644
--- a/plugins/symbol-tree/gbp-symbol-layout-stack-addin.c
+++ b/plugins/symbol-tree/gbp-symbol-layout-stack-addin.c
@@ -243,7 +243,13 @@ gbp_symbol_layout_stack_addin_get_symbol_tree_cb (GObject      *object,
 
   /* If we were destroyed, short-circuit */
   if (self->button != NULL)
-    gbp_symbol_menu_button_set_symbol_tree (self->button, tree);
+    {
+      /* Only override if we got a new value (this helps with situations
+       * where the parse tree breaks intermittently.
+       */
+      if (tree != NULL)
+        gbp_symbol_menu_button_set_symbol_tree (self->button, tree);
+    }
 }
 
 static void