Be more careful about NULL pointers.

I really meant to write this code in the last commit, but the cat
distracted me by jumping on the keyboard and I forgot to do it.

1 files changed, 6 insertions, 3 deletions

diff --git a/src/generic/problemresolver/incremental_expression.h b/src/generic/problemresolver/incremental_expression.h
index 4b6b73c1..7c8fa9c5 100644
--- a/src/generic/problemresolver/incremental_expression.h
+++ b/src/generic/problemresolver/incremental_expression.h
@@ -276,14 +276,17 @@ public:
 
   ~expression_box()
   {
-    child->remove_parent(this);
+    if(child.valid())
+      child->remove_parent(this);
   }
 
   void set_child(const cwidget::util::ref_ptr<expression<T> > &new_child)
   {
-    child->remove_parent(this);
+    if(child.valid())
+      child->remove_parent(this);
     child = new_child;
-    new_child->add_parent(this);
+    if(new_child.valid())
+      new_child->add_parent(this);
   }
 
   const cwidget::util::ref_ptr<expression<T> > &get_child() const