panic during device delete

{"level":"info","ts":"2022-12-06T19:46:59.740Z","caller":"core/openolt.go:243","msg":"delete-device","instanceId":"voltha-voltha-adapter-openolt-5ffd7cf655-xt59w","op-name":"/voltha.VolthaService/DeleteDevice","task-id":"7b1836dbe3151596","task-name":"/adapter_service.AdapterService/DeleteDevice","device-id":"3cba38ce-dac8-429c-9bc9-25ed96c60b89","op-id":"13d9a56324b82df6","device-id":"3cba38ce-dac8-429c-9bc9-25ed96c60b89"} {"level":"debug","ts":"2022-12-06T19:46:59.740Z","caller":"core/device_handler.go:2166","msg":"function-entry-delete-device","instanceId":"voltha-voltha-adapter-openolt-5ffd7cf655-xt59w","task-id":"7b1836dbe3151596","task-name":"/adapter_service.AdapterService/DeleteDevice","device-id":"3cba38ce-dac8-429c-9bc9-25ed96c60b89","op-id":"13d9a56324b82df6","op-name":"/voltha.VolthaService/DeleteDevice","device-id":"3cba38ce-dac8-429c-9bc9-25ed96c60b89"}

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x68 pc=0xeb7257]

goroutine 14297 [running]:
github.com/opencord/voltha-openolt-adapter/internal/pkg/core.(*OpenOltFlowMgr).StopAllFlowHandlerRoutines(0x0, 0x12fad40, 0xc0021c5ef0, 0xc00058f370)
/go/src/internal/pkg/core/openolt_flowmgr.go:2308 +0x37
created by github.com/opencord/voltha-openolt-adapter/internal/pkg/core.(*DeviceHandler).DeleteDevice
/go/src/internal/pkg/core/device_handler.go:2178 +0x256