This has been an issue for at least a couple of years. Worlds from around 2016 or earlier cause the game to crash on "Building Terrain". I tried to load it on Android and received the same bug. This has been a problem with multiple different older worlds in the past.

To reproduce:

Try to load a world from an older version of Minecraft Pocket Edition in modern versions - It seems to be worlds prior to around version 1.0.0 that have the problem - a copy of my world from January 2017 loads fine, but a copy from December 2016 crashes.

Edit: Interestingly, a world from earlier in 2016 loaded successfully, which was a pleasant surprise. Doesn't seem like all worlds are affected by this?

Edit 2: I suspect this issue has been significantly improved from the issues I was having a while ago. Most of the worlds loaded successfully and only one failed. I wrongly assumed from one world that it was still a bug affecting all old worlds (as it was before). But still, one world out of six crashed, which is still probably an issue.