Update label of window form only if it exists

The update of label was introduced in 89834116fe97 but the label is defined
only when the window form is created with the view type tree.

Closes #13994

sao/src/window.js

@@ -416,7 +416,9 @@
                 }
             }
             let message = name + '/' + Sao.common.humanize(size);
-            this.label.text(message).attr('title', message);
+            if (this.label) {
+                this.label.text(message).attr('title', message);
+            }
             this._set_button_sensitive();
         },
         record_modified: function() {

tryton/tryton/gui/window/win_form.py

@@ -365,7 +365,8 @@ class WinForm(NoModal, InfoBar):
         if selected > 1:
             name += '#%i' % selected
         name = '(%s/%s)' % (name, common.humanize(size))
-        self.label.set_text(name)
+        if hasattr(self, 'label'):
+            self.label.set_text(name)
         self._set_button_sensitive()
 
     def record_modified(self, *args):