Fix #3639, KeyError on Preview Link (#3663)

If the session key does not exist, fall back to the preview error.  The user can then try previewing the article again.

wagtail/wagtailadmin/tests/test_pages_views.py

@@ -1550,6 +1550,29 @@ class TestPageEdit(TestCase, WagtailTestUtils):
         self.assertTemplateUsed(response, 'tests/simple_page.html')
         self.assertContains(response, "I've been edited!")
 
+    def test_preview_on_edit_no_session_key(self):
+        preview_url = reverse('wagtailadmin_pages:preview_on_edit',
+                              args=(self.child_page.id,))
+
+        # get() without corresponding post(), key not set.
+        response = self.client.get(preview_url)
+
+        # Check the HTML response
+        self.assertEqual(response.status_code, 200)
+
+        # We should have an error page because we are unable to
+        # preview; the page key was not in the session.
+        self.assertContains(
+            response,
+            "<title>Wagtail - Preview error</title>",
+            html=True
+        )
+        self.assertContains(
+            response,
+            "<h1>Preview error</h1>",
+            html=True
+        )
+
     @modify_settings(ALLOWED_HOSTS={'append': 'childpage.example.com'})
     def test_preview_uses_correct_site(self):
         # create a Site record for the child page

wagtail/wagtailadmin/views/pages.py

@@ -573,6 +573,10 @@ class PreviewOnEdit(View):
         page = self.get_page()
         form_class = page.get_edit_handler().get_form_class(page._meta.model)
         parent_page = page.get_parent().specific
+
+        if self.session_key not in self.request.session:
+            # Session key not in session, returning null form
+            return form_class(instance=page, parent_page=parent_page)
         post_data_dict, timestamp = self.request.session[self.session_key]
 
         # convert post_data_dict back into a QueryDict