Prevent panic to run streaming_wordcap

$ cd beam/sdks/go/examples/streaming_wordcap/wordcap.go
$ go run wordcap.go \
  --project=MY_PROJECT\
  --temp_location=gs://MY_BUKET/tmp\
  --staging_location=gs://MY_BUKET/binaries\
  --runner=dataflow
...
panic: runtime error: invalid memory address or nil pointer dereferencepanic: runtime error: invalid memory address or nil pointer dereference[signal SIGSEGV: segmentation violation code=0x1 addr=0x58 pc=0x15b85c3]
goroutine 1 [running]:github.com/apache/beam/sdks/go/pkg/beam/core/runtime/graphx.(*marshaller).addMultiEdge(0xc00079ba10, 0x1d16d9d, 0x8, 0xc00025a0a0, 0x2544dc0, 0xc0001f6c00, 0x2, 0x30, 0x2867560)
        $GOPATH/pkg/mod/github.com/apache/beam@v2.2.1-0.20210111102611-0994dfe6faef+incompatible/sdks/go/pkg/beam/core/runtime/graphx/translate.go:305 +0x163
...