minor #863 Fixed the handling of flash messages (javiereguiluz)

This PR was merged into the master branch.

Discussion
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Fixed the handling of flash messages

Fixes the #859 merge.

Commits
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e6349dc Fixed the handling of flash messages

templates/default/_flash_messages.html.twig

@@ -5,15 +5,16 @@
    A common practice to better distinguish between templates and fragments is to
    prefix fragments with an underscore. That's why this template is called
    '_flash_messages.html.twig' instead of 'flash_messages.html.twig'
+#}
 
-   We check if we have session before reading flashes as it otherwise triggers session start:
+{#
+   The request method check is needed to prevent starting the session when looking for "flash messages":
    https://symfony.com/doc/current/session/avoid_session_start.html
 
    TIP: With FOSHttpCache you can also adapt this to make it cache safe:
    https://foshttpcachebundle.readthedocs.io/en/latest/features/helpers/flash-message.html
 #}
-
-{% if app.request.hasPreviousSession %}
+{% if app.request.method == 'POST' %}
     <div class="messages">
         {% for type, messages in app.flashes %}
             {% for message in messages %}