erp5_crm: display CRM "preview" view on Letters only if erp5_interface_post isn't installed

bt5/erp5_crm/ActionTemplateItem/portal_types/Letter/preview.xml

@@ -26,11 +26,13 @@
         </item>
         <item>
             <key> <string>condition</string> </key>
-            <value> <string></string> </value>
+            <value>
+              <persistent> <string encoding="base64">AAAAAAAAAAM=</string> </persistent>
+            </value>
         </item>
         <item>
             <key> <string>description</string> </key>
-            <value> <string></string> </value>
+            <value> <string>Default preview action for Events</string> </value>
         </item>
         <item>
             <key> <string>icon</string> </key>
@@ -76,4 +78,17 @@
       </dictionary>
     </pickle>
   </record>
+  <record id="3" aka="AAAAAAAAAAM=">
+    <pickle>
+      <global name="Expression" module="Products.CMFCore.Expression"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>text</string> </key>
+            <value> <string>python: getattr(context.getPortalObject(), \'letter_message_post_module\', None) is None</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
 </ZopeData>