Hi Joseph_S
Tested in safe mode and with another user account and the issue didn't happen.
I have also tried to find the file/app in order to delete but cannot find as it is likely very hidden. Also opened force quit but only safari open and even when safari is not open at all the box keeps popping up.
When I click report it shows more info such as:
Process: SystemExtr [1520]
Path: /Users/USER/Library/SystemExtr/SystemExtr.app/Contents/MacOS/SystemExtr
Identifier: SystemExtr.SystemExtr
Version: 1.0 (1)
Code Type: X86-64 (Native)
Parent Process: ??? [1]
Responsible: SystemExtr [1520]
User ID: 501
Date/Time: 2018-12-22 09:06:52.998 +1000
OS Version: Mac OS X 10.14.2 (18C54)
Report Version: 12
Anonymous UUID: 8B44F3FB-C0E1-C683-23A7-41303C66C973
Time Awake Since Boot: 590 seconds
System Integrity Protection: enabled
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_BAD_INSTRUCTION (SIGILL)
Exception Codes: 0x0000000000000001, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Termination Signal: Illegal instruction: 4
Termination Reason: Namespace SIGNAL, Code 0x4
Terminating Process: exc handler [1520]
Application Specific Information:
Fatal error: 'try!' expression unexpectedly raised an error: CryptoSwift.AES.Error.invalidData: file /BuildRoot/Library/Caches/com.apple.xbs/Sources/swiftlang_Fall2018/swiftlang_Fall2018-1000.11.42/src/swift/stdlib/public/core/ErrorType.swift, line 184