Fix error case where the tour start page is skipped

This was causing us to key error for 'placeholders' being missing in the
session dictionary. This fixes it and sends you back to the start

app/main/views/tour.py

@@ -49,6 +49,11 @@ def tour_step(service_id, template_id, step_index):
     if db_template['template_type'] != 'sms':
         abort(404)
 
+    if 'placeholders' not in session:
+        return redirect(url_for(
+            '.begin_tour', service_id=current_service.id, template_id=template_id
+        ))
+
     template = get_template(
         db_template,
         current_service,
@@ -119,6 +124,11 @@ def check_tour_notification(service_id, template_id):
         show_recipient=True,
     )
 
+    if 'placeholders' not in session:
+        return redirect(url_for(
+            '.begin_tour', service_id=current_service.id, template_id=template_id
+        ))
+
     placeholders = fields_to_fill_in(template, prefill_current_user=True)
 
     if not all_placeholders_in_session(template.placeholders):

tests/app/main/views/test_tour.py

@@ -234,6 +234,30 @@ def test_should_403_if_user_does_not_have_send_permissions_for_tour_step(
     )
 
 
+def test_tour_step_redirects_to_tour_start_if_placeholders_doesnt_exist_in_session(
+    client_request,
+    mock_get_service_template_with_multiple_placeholders,
+    service_one,
+    fake_uuid,
+):
+    with client_request.session_transaction() as session:
+        assert 'placeholders' not in session
+
+    client_request.get(
+        'main.tour_step',
+        service_id=SERVICE_ONE_ID,
+        template_id=fake_uuid,
+        step_index=1,
+        _expected_status=302,
+        _expected_redirect=url_for(
+            'main.begin_tour',
+            service_id=SERVICE_ONE_ID,
+            template_id=fake_uuid,
+            _external=True,
+        ),
+    )
+
+
 def test_back_link_from_first_get_tour_step_points_to_tour_start(
     client_request,
     mock_get_service_template_with_multiple_placeholders,
@@ -518,6 +542,30 @@ def test_back_link_from_check_tour_notification_points_to_last_tour_step(
     )
 
 
+def test_check_tour_notification_redirects_to_tour_start_if_placeholders_doesnt_exist_in_session(
+    client_request,
+    mock_get_service_template_with_multiple_placeholders,
+    service_one,
+    fake_uuid,
+):
+    with client_request.session_transaction() as session:
+        assert 'placeholders' not in session
+
+    client_request.get(
+        'main.check_tour_notification',
+        service_id=SERVICE_ONE_ID,
+        template_id=fake_uuid,
+        step_index=1,
+        _expected_status=302,
+        _expected_redirect=url_for(
+            'main.begin_tour',
+            service_id=SERVICE_ONE_ID,
+            template_id=fake_uuid,
+            _external=True,
+        ),
+    )
+
+
 def test_check_tour_notification_redirects_to_first_step_if_not_all_placeholders_in_session(
     client_request,
     mock_get_service_template_with_multiple_placeholders,