How to reproduce:
1. Open attached project
2. Enter the Play Mode
3. Press 'Start System Group' button
4. Press 'Start System' button
5. Press 'StopSystem Group' button
6. Observer memory leak detected in console.

Warnings:
"To Debug, enable the define: TLA_DEBUG_STACK_LEAK in ThreadsafeLinearAllocator.cpp. This will output the callstacks of the leaked allocations"
"Internal: JobTempAlloc has allocations that are more than 4 frames old - this is not allowed and likely a leak"

Reproducible with: 2019.1.0b10, 2019.2.0a10
Not reproducible with: 2017.4 no Entities, 2018.3.12f1 due to "ComponentSystemGroup" errors

Actual results: SystemGroup doesn't get disabled, and leaks memory
Expected results: SystemGroup should be disabled, no warnings should be present