nil pointer dereference when using type: FromCompositeFieldPath and omitting valueFromFieldPath

[sschne]

What happened?

When adding a pipeline step that uses selector with FromCompositeFieldPath and omitting valueFromFieldPath, the function crashes with nil pointer dereference

panic: runtime error: invalid memory address or nil pointer dereference

[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x13e4292]

goroutine 7 [running]:
main.buildRequirements(0xc000244780, 0xc000225130)
        /fn/fn.go:140 +0x4d2
main.(*Function).RunFunction(0xc00040c7f0, {0xc0006800c0?, 0x499ae6?}, 0xc0006800c0)
        /fn/fn.go:57 +0x265
github.com/crossplane/function-sdk-go/proto/v1beta1._FunctionRunnerService_RunFunction_Handler({0x1506580?, 0xc00040c7f0}, {0x190c0a0, 0xc0001ee930}, 0xc00050c080, 0x0)
        /go/pkg/mod/github.com/crossplane/[email protected]/proto/v1beta1/run_function_grpc.pb.go:104 +0x169
google.golang.org/grpc.(*Server).processUnaryRPC(0xc0001b6e00, {0x190c0a0, 0xc0001ee690}, {0x1911900, 0xc0001f6680}, 0xc0000ec000, 0xc0000afb60, 0x23f39c0, 0x0)
        /go/pkg/mod/google.golang.org/[email protected]/server.go:1385 +0xe03
google.golang.org/grpc.(*Server).handleStream(0xc0001b6e00, {0x1911900, 0xc0001f6680}, 0xc0000ec000)
        /go/pkg/mod/google.golang.org/[email protected]/server.go:1796 +0xfec
google.golang.org/grpc.(*Server).serveStreams.func2.1()
        /go/pkg/mod/google.golang.org/[email protected]/server.go:1029 +0x8b
created by google.golang.org/grpc.(*Server).serveStreams.func2 in goroutine 24
        /go/pkg/mod/google.golang.org/[email protected]/server.go:1040 +0x135

How can we reproduce it?

apiVersion: apiextensions.crossplane.io/v1
kind: Composition
metadata:
  name: function-environment-configs
spec:
  compositeTypeRef:
    apiVersion: example.crossplane.io/v1
    kind: XR
  mode: Pipeline
  pipeline:
  - step: pull-extra-resources
    functionRef:
      name: function-extra-resources
    input:
      apiVersion: extra-resources.fn.crossplane.io/v1beta1
      kind: Input
      spec:
        extraResources:
          - kind: XCluster
            into: XCluster
            apiVersion: example.crossplane.io/v1
            type: Selector
            selector:
              maxMatch: 2
              minMatch: 1
              matchLabels:
                - key: type
                  type: FromCompositeFieldPath
                  value: cluster

What environment did it happen in?

Function version: 0.0.3