Symptoms

1. Undefined pages can be seen as such from Browse >> Pages >> Page Tree:
   Page 1
   Page 2
   Child Page 2
   Page 3
   Undefined page
   Undefined page
2. Deleting the undefined / untitled pages causes a NullPointerException.

Cause

The content has been corrupted while upgrading.

Resolution

1. Backup the space using XML
2. Delete the space
3. Edit the entities.xml file by deleting the following references in the backup zip file to remove these pages. References (check out the pageid first):

   
   <object class="BodyContent" package="com.atlassian.confluence.core">
   ...
   <property name="content" class="Page" package="com.atlassian.confluence.pages"><id name="id">1507337</id>
   ...
   </object>
   
   <object class="Page" package="com.atlassian.confluence.pages">
   <id name="id">1507337</id>
   ...
   </object>
   
   <object class="BucketPropertySetItem" package="bucket.user.propertyset">
   <composite-id><property name="entityName"><![CDATA[confluence_ContentEntityObject]]></property>
   <property name="entityId">1507337</property>
   ...
   <property name="dateVal"/></object>
   

4. Finally restore the edited backup file