[gtk/issue-2900: 1/2] Annotate nullable argument

[Emmanuele Bassi <ebassi src gnome org>]

commit 6a4b5ead86749c1073ca5ac2e83a443f40970b24
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Sun Jun 28 13:39:17 2020 +0100

    Annotate nullable argument
    
    The `this_` argument is passed to `gtk_expression_watch()`, which
    accepts it as nullable.
    
    Fixes: #2900

 gtk/gtkexpression.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/gtkexpression.c b/gtk/gtkexpression.c
index d690487ca8..750ee2c57c 100644
--- a/gtk/gtkexpression.c
+++ b/gtk/gtkexpression.c
@@ -1961,7 +1961,7 @@ gtk_expression_bind_notify (gpointer data)
  * @self: (transfer full): a #GtkExpression
  * @target: (transfer none) (type GObject): the target object to bind to
  * @property: name of the property on @target to bind to
- * @this_: (transfer none) (type GObject): the this argument for
+ * @this_: (transfer none) (type GObject) (nullable): the this argument for
  *     the evaluation of @self
  *
  * Bind @target's property named @property to @self.