ORA 00054 in oracle forms

Hi,

I have a forms, which displays the list of orders, this list is populated form a view.
When a users selects one an selects an option to update it, based on the option the respective forms is called and the order number is passed.
The order is queried from the table in the called forms i.e, th e child form. Everything is working fine, but if an another user is updated the same record its giving an error ORA 00054 .
Now i would like to display an error message that it is not update able and redirect back to parent forms.
How do i handle this error,
i have tried it in the block where i am executing the query but it doesn't works.

Thanks