I keep getting this critical error.

[2016.07.18-20.54.37:685][150]LogWindows:Error: === Critical error: ===
Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:C:\Program Files (x86)\Epic Games\4.12\Engine\Source\Runtime\Core\Public\Containers\Array.h] [Line: 804]
Array index out of bounds: 2 from an array of size 2

I’m cautious when dealing with the Engine’s source code, so I just want to be sure if this error means the problem lies there.

No. This error means you are trying to access an index of an array, declared somewhere, which surpasses the actual size of the array during the time the index was given for read.
Like, if you have array of size 2, only possible indexes are 0 and 1; not 2 which implies an array of size 3 at least…

You can start the engine in the debug mode through Visual Studio and when the editor crashes it will tell you why it crashed and when it crashed.
I think it is not the problem in the Unreal’s source code but probably the problem is in your code.
I had this error when I was trying to get an element from the Array that was not valid but I was not aware of it, you should always do a check before getting something from the Array if you are not sure if it is going to be valid.