Conditionally load the zcml for our portlets.

Only load them when plone.app.portlets is available and Classic UI is wanted.
This is the default in Plone 6, but may be switched off in 6.1.
In Plone 7 this won't be the default.

See plone/Products.CMFPlone#3923

news/3923.bugfix

@@ -0,0 +1,2 @@
+Conditionally load the zcml for our portlets.
+[maurits]

plone/app/event/configure.zcml

@@ -12,19 +12,23 @@
   <include package="plone.event" />
   <include package="plone.resource" />
   <include package="plone.formwidget.recurrence" />
-  <include package="plone.app.portlets" />
   <include package="plone.app.registry" />
   <include package="plone.app.z3cform" />
 
   <!-- internal dependencies -->
   <include file="permissions.zcml" />
   <include package=".dx" />
   <include package=".browser" />
-  <include package=".portlets" />
   <include package=".ical" />
   <include file="recurrence.zcml" />
   <include package=".upgrades" />
 
+  <configure zcml:condition="not-have disable-classic-ui">
+    <configure zcml:condition="installed plone.app.portlets">
+      <include package="plone.app.portlets" />
+      <include package=".portlets" />
+    </configure>
+  </configure>
   <utility
       name="plone.app.event.SynchronizationStrategies"
       component=".vocabularies.SynchronizationStrategies"