mongo package error

Req: headers:<key:"X-Droi-AppID" value:"bf8umbzhigY-zYLSzOhEifAD91gTRXU2lQAAGsIK" > headers:<key:"_Id" value:"01e9d0188edee524cff7d101" > headers:<key:"body" value:"{\"db\": \"DroiObjectApi\", \"role\":\"56cea1d8c9922205b5bdfab7\", \"uid\":\"test\", \"pwd\":\"test\"}" > headers:<key:"collection" value:"DroiObjectApi" > headers:<key:"path" value:"unitest" > headers:<key:"role" value:"56cea1d8c9922205b5bdfab7" > headers:<key:"version" value:"1" > 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4a93b9]

goroutine 18166 [running]:
panic(0xb0fd00, 0xc42000c0c0)
        /home/tony/go/go/src/runtime/panic.go:500 +0x1a1
github.com/DroiTaipei/mongo.getReadSess(0x7f761d80b960, 0x0, 0xc41fa2fd05)
        /home/tony/code/go_buster/vendor/src/github.com/DroiTaipei/mongo/mongo.go:96 +0x69
github.com/DroiTaipei/mongo.QueryAll(0xc4278ecf80, 0x3, 0x4, 0xc42f435e40, 0x3d, 0xab1360, 0xc429597ea0, 0xb17180, 0xc42ba05e00, 0xb17180, ...)
        /home/tony/code/go_buster/vendor/src/github.com/DroiTaipei/mongo/access.go:258 +0x5b
api.GetGroupIDs(0xc4278ecf80, 0x3, 0x4, 0xc42ba05d70, 0x28, 0xc429597e20, 0x18, 0xc420186600, 0x0, 0x0, ...)
        /home/tony/code/go_buster/src/api/classes.go:952 +0x4d2
api.DocumentPreChecker(0xc429ef7c80, 0x47c8f3, 0xb0c7c0)
        /home/tony/code/go_buster/src/api/classes.go:160 +0x6b7
api.DocumentFindHandler(0xc429ef7c80, 0xc42ba05d10, 0xbc87d3, 0x5, 0xf426a0, 0x0, 0xc431a43a90)
        /home/tony/code/go_buster/src/api/classes.go:216 +0x2f
grpcapi.(*Server).DocumentRead(0xc4205a8000, 0xefa600, 0xc42ba05aa0, 0xc429597e00, 0x0, 0x119, 0x119)
        /home/tony/code/go_buster/src/grpcapi/api.go:69 +0x392
grpcapi/protobuf/mongo._Mongo_DocumentRead_Handler(0xba8500, 0xc4205a8000, 0xefa600, 0xc42ba05aa0, 0xc43005fb80, 0x0, 0x0, 0x0, 0x2, 0xc425a793c0)
        /home/tony/code/go_buster/src/grpcapi/protobuf/mongo/mongo.pb.go:386 +0x27d
google.golang.org/grpc.(*Server).processUnaryRPC(0xc42027c000, 0xefcc00, 0xc420090ea0, 0xc42ee21c20, 0xc4204b0180, 0xf1a6e8, 0xc42ba05980, 0x0, 0x0)
        /home/tony/code/go_buster/vendor/src/google.golang.org/grpc/server.go:611 +0xc14
google.golang.org/grpc.(*Server).handleStream(0xc42027c000, 0xefcc00, 0xc420090ea0, 0xc42ee21c20, 0xc42ba05980)
        /home/tony/code/go_buster/vendor/src/google.golang.org/grpc/server.go:769 +0x6b0
google.golang.org/grpc.(*Server).serveStreams.func1.1(0xc4205e9660, 0xc42027c000, 0xefcc00, 0xc420090ea0, 0xc42ee21c20)
        /home/tony/code/go_buster/vendor/src/google.golang.org/grpc/server.go:422 +0xab
created by google.golang.org/grpc.(*Server).serveStreams.func1
        /home/tony/code/go_buster/vendor/src/google.golang.org/grpc/server.go:423 +0xa3