Load plone.app.discussion code conditionally.

This package is optional since Plone 6.1.

CHANGES.rst

@@ -4,6 +4,9 @@ Changelog
 3.2.0 (unreleased)
 ------------------
 
+- Load ``plone.app.discussion`` code conditionally.
+  This package is optional since Plone 6.1.  [maurits]
+
 - existingcontent tile:
   Introduce new "main" macro lookup to enable full layout control within
   registered "content_views" templates.

src/plone/app/standardtiles/existingcontent.py

@@ -3,7 +3,6 @@
 from plone import api
 from plone.app.blocks import utils
 from plone.app.blocks.tiles import renderTiles
-from plone.app.discussion.interfaces import IConversation
 from plone.app.standardtiles import PloneMessageFactory as _
 from plone.autoform import directives as form
 from plone.base.utils import safe_text
@@ -38,6 +37,12 @@
         RelatedItemsFieldWidget as ExistingContentBrowserWidget,
     )
 
+try:
+    from plone.app.discussion.interfaces import IConversation
+except ImportError:
+    # plone.app.discussion is optional since Plone 6.1.
+    IConversation = None
+
 
 def uuidToObject(uuid):
     """Given a UUID, attempt to return a content object. Will return
@@ -267,6 +272,8 @@ def image_tag(self):
 
     @property
     def comments_count(self):
+        if IConversation is None:
+            return 0
         context = self.content_context
         try:
             conversation = IConversation(context)

src/plone/app/standardtiles/layout.zcml

@@ -173,6 +173,7 @@
       class=".discussion.DiscussionTile"
       permission="zope.Public"
       add_permission="zope2.View"
+      zcml:condition="installed plone.app.discussion"
       />
 
 </configure>