WhatsApp crashes while loading the previous messages in the chat history. This happens randomly for any chat history. This bug has been reported by almost everyone who has Windows Phone 8 or Lumia devices. WhatsApp has not yet fixed this bug. Hopefully it will look into this and come up with an update with the fix.
I have Nokia Lumia 620 with Amber update and latest WhatsApp.
Not many would know that there is a small workaround to this problem. This workaround works with patience. :)
All you need to do is swipe down to load the chat history, or the old messages, only ONCE. And wait for the old messages to load (You will know that the old messages has loaded, once the loading dots of windows phone stops). After the messages get loaded, swipe down once again and you will be able to read the old messages. This way the WhatsApp wont crash.
That is why elders always say "Have some Patience".
If this trick worked for you, please comment with the phone model, so that we know if it really works for others too.