Changes to a Dynamic Action with a Client-side condition of type “Item is null” from pre 24.2 APEX pages causes an error when trying to save the page stating someone else has made a change to this page, reload the page, losing your changes.
The problem is the Client-side condition types changed from “Item is null” prior to 24.2 to “Item/column is null” in 24.2 (Similar problem for all the other conditions). So, when you make any change to the Dynamic Action APEX “fixes” the client-side condition by changing it to "Select" and wiping out the condition because it thinks the Item type is invalid.
This is a HUGE problem, especially since there is a different issue with Custom Events not firing on JavaScript Expression “Document” conditions anymore, requiring the change of many Dynamic Actions….. (I did a separate post for this issue earlier today.)