Clear attachment preview when there is no current record

Closes #15044
(grafted from dc0a158ea439898e071602e8dc87fc4767fe76c6)

--HG--
branch : 8.0

sao/src/tab.js

@@ -1398,10 +1398,9 @@
                 return;
             }
             var record = this.screen.current_record;
-            if (!record) {
-                return;
-            }
-            var resource = record.model.name + ',' + record.id;
+            var resource = (
+                record ? record.model.name + ',' + record.id :
+                this.screen.model_name + ',-1');
             var domain = [
                 ['resource', '=', resource],
                 ['type', '=', 'data'],

tryton/tryton/gui/window/form.py

@@ -245,9 +245,9 @@ class Form(TabContent):
         if not self.attachment_screen:
             return
         record = self.screen.current_record
-        if not record:
-            return
-        resource = '%s,%s' % (record.model_name, record.id)
+        resource = '%s,%s' % (
+            (record.model_name, record.id) if record
+            else (self.screen.model_name, -1))
         domain = [
             ('resource', '=', resource),
             ('type', '=', 'data'),