crash using $exists

[nikhan]

panic: interface conversion: interface is nil, not map[string]interface {}

goroutine 255055 [running]:
runtime.panic(0x94f020, 0xc210a1e7c0)
/usr/local/go/src/pkg/runtime/panic.c:266 +0xb6
github.com/nytlabs/gojee.func·043(0x0, 0x0, 0x865de0, 0xc210c48e50, 0xc210052248, ...)
/home/ubuntu/go/src/github.com/nytlabs/gojee/jee.go:788 +0xb2
github.com/nytlabs/gojee.Eval(0xc2109b6900, 0x863380, 0xc210438900, 0x864700, 0x0, ...)
/home/ubuntu/go/src/github.com/nytlabs/gojee/jee.go:1107 +0x2ad2
github.com/nytlabs/gojee.Eval(0xc2109b6880, 0x863380, 0xc210438900, 0x864700, 0x1, ...)
/home/ubuntu/go/src/github.com/nytlabs/gojee/jee.go:983 +0x83f
github.com/nytlabs/gojee.Eval(0xc2109b6600, 0x863380, 0xc210438900, 0xc2103cd701, 0x1, ...)
/home/ubuntu/go/src/github.com/nytlabs/gojee/jee.go:1112 +0x2d24
github.com/nytlabs/streamtools/st/library.(*Filter).Run(0xc2105f25b0)
/home/ubuntu/go/src/github.com/nytlabs/streamtools/st/library/filter.go:46 +0x151
created by github.com/nytlabs/streamtools/st/blocks.BlockRoutine
/home/ubuntu/go/src/github.com/nytlabs/streamtools/st/blocks/blocks.go:244 +0xfb