A globally-managed aggregation cursor can be established even if the database over which the aggregation is issued does not exist. On a subsequent OP_GET_MORE, a check is made that the aggregation namespace is not a view (since OP_GET_MORE on views is not supported):

https://github.com/mongodb/mongo/blob/ab165e7a81e319cd7e99af3e1eed86e826fd34ba/src/mongo/db/query/find.cpp#L281-L287

However, this code incorrectly assumes that the Database object exists. If it doesn't exist, getDb() will return null, causing this line to deference a null pointer and crash the server.

This issue was introduced during 3.5 development and does not affect any stable versions of MongoDB.