https://goo.gl/yRCHtg

Can you show me your spawn.json?

[
{
"minheight": 50,
"hostile": true,
"result": "deny"
},
{
"maxheight": 50,
"block": ["minecraft:stone", "minecraft:cobblestone"],
"result": "allow"
},
{
"result": "deny"
}
]

I'll add some crash protection

Fixed next release