what does Error "Thread 1:EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)" mean?
This line
secondNumber = Screen.text!.toInt()!means: Get the Screen object, get the text property and please crash if it doesn't exist, then get the text converted to an integer, and please crash if it doesn't exist.
That's what the ! means: "I am sure this thing exists, so please crash if it doesn't". And crash is what it did.
Generally, EXC_BAD_INSTRUCTION means that there was an assertion failure in your code. A wild guess, your Screen.text is not an integer. Double check its type.
Mine was about
dispatch_group_leave(group)
was inside if closure in block. I just moved it out of closure.