BBSIM crashed when adding flows

BBSIM crashes with below stack trace

recv EchoReq
time="2023-07-31T18:52:45.606561271Z" level=info msg="OLT receives GetDeviceInfo call from VOLTHA" AllocIdEnd=1152 AllocIdStart=1024 DeviceId="0a:0a:0a:0a:0a:03" DeviceSerialNumber=BBSIMOLT3 FirmwareVersion= FlowIdEnd=2 FlowIdStart=1 GemportIdEnd=2048 GemportIdStart=1024 HardwareVersion=emulated Model=asfvolt16 OnuIdEnd=32 OnuIdStart=1 PonPorts=16 PreviouslyConnected=false Technology=XGS-PON Vendor=BBSim module=OLT
time="2023-07-31T18:52:45.610691421Z" level=debug msg="Received upstream single tagged flow"
time="2023-07-31T18:52:45.610888319Z" level=debug msg="Created openflow flow &{

4311777808 0 0 0 0 0 100 4294967295 4294967295 4294967295 0 0xc003f27fe0 [0xc00401e040 0xc003fcb098]}"
time="2023-07-31T18:52:45.610951274Z" level=debug msg="Created openflolw match &

"
time="2023-07-31T18:52:45.610982403Z" level=debug msg="Created openflow instructions [0xc00401e280]"
time="2023-07-31T18:52:45.611029093Z" level=debug msg="Created openflow flow &{

4311777808 4503599627370495 1 0 0 0 100 4294967295 4294967295 4294967295 0 0xc00401e200 [0xc00401e280]}"
time="2023-07-31T18:52:45.611086687Z" level=error msg=trying-to-store-gemport-for-unknown-onu FlowId=1342485023589485499 GemportId=1031 IntfId=1 OnuId=1 PortNo=65792 module=OLT
panic: assignment to entry in nil map

goroutine 108 [running]:
github.com/opencord/bbsim/internal/bbsim/devices.(*OltDevice).storeGemPortId(0x1a60f60, 0x100000001, 0x40700010100, 0x12a1770423554bbb)
        /app/internal/bbsim/devices/olt.go:1757 +0x786
github.com/opencord/bbsim/internal/bbsim/devices.(*OltDevice).storeGemPortIdByFlow(0x1a60f60, 0xc00003fcc0)
        /app/internal/bbsim/devices/olt.go:1778 +0x492
github.com/opencord/bbsim/internal/bbsim/devices.(*OltDevice).FlowAdd(0x1a60f60, 0x12821c0, 0xc002c22030, 0xc00003fcc0, 0x1a60f60, 0xc002c22030, 0xc001ca9a80)
        /app/internal/bbsim/devices/olt.go:1133 +0xe23
github.com/opencord/voltha-protos/v5/go/openolt._Openolt_FlowAdd_Handler(0x1069a00, 0x1a60f60, 0x12821c0, 0xc002c22030, 0xc003f4daa0, 0x0, 0x12821c0, 0xc002c22030, 0xc003fc2360, 0x113)
        /app/vendor/github.com/opencord/voltha-protos/v5/go/openolt/openolt.pb.go:5675 +0x217
google.golang.org/grpc.(*Server).processUnaryRPC(0xc000513200, 0x12c1c00, 0xc0004dbe00, 0xc003fac600, 0xc0035ade30, 0x1a52d00, 0x0, 0x0, 0x0)
        /app/vendor/google.golang.org/grpc/server.go:1007 +0x460
google.golang.org/grpc.(*Server).handleStream(0xc000513200, 0x12c1c00, 0xc0004dbe00, 0xc003fac600, 0x0)
        /app/vendor/google.golang.org/grpc/server.go:1287 +0xd99
google.golang.org/grpc.(*Server).serveStreams.func1.1(0xc0044bfc20, 0xc000513200, 0x12c1c00, 0xc0004dbe00, 0xc003fac600)
        /app/vendor/google.golang.org/grpc/server.go:722 +0xbb
created by google.golang.org/grpc.(*Server).serveStreams.func1
        /app/vendor/google.golang.org/grpc/server.go:720 +0xa1