While BlackBerry is working on a fix for this oversight, there is a workaround. Once you launch BBM for iOS, instead of pressing a contact name under Chats, which causes the app to crash, a long tap on a contact name will reveal a slide out screen appearing from the right side. On the bottom of the slide out screen is a trash can icon which is used to delete the entire thread involving the contact selected. Press on the icon to bring up a window that offers you an option to "End Chat". The hard work is now done. Go to the Contacts tab, click on the name of the person you want to communicate with, and you should be all good to go!
Another issue affecting BBM for Android is a number of glowing reviews for the app in the Google Play Store that all read, "Thank you so much blackberry team. I was waiting this app. Its really great user friendly and smooth." BlackBerry says that it is aware of the issue and is working with Google to "resolve" it.
Workaround to prevent crashing of BBM for iOS
source: RedmondPie, AndroidCentral