i just did a command and it worked but then i just changed one thing and it crashed this is what i did....

/setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {SpawnPotentials:[{Weight:1,Entity:{id:"Skeleton"}}]}

then i changed it like this....

/setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {SpawnPotentials:[{Weight:0,Entity:{id:"Skeleton"}}]}