Exception in server tick loop

MC has said "run out of memory" and shows the options of "return to main menu" and "quit game" and it does that after around 20, maybe 30 minutes of game play.

What I expected to happen was...:
To keep playing normally like on my other laptop

What actually happened was...:
I was playing without any lag, and just before it'd show the error, it would start to lag a little, freeze then show the error.

Steps to Reproduce:
1. play for 20, 30 minutes as if nothing's going to happen on map "A"
2. MC shows error
3. hit "return to main menu"
4. single player and select the same map
5. few seconds to a few minutes later after opening the same map without closing MC after the first time, MC shows the error page again.
--If you close MC after the first time it'll last 20 to 30 minutes still.