Fix panic error when target collection is not exist.
Fix panic error when target collection is not exist.

Fix: interface conversion: interface {} is nil, not *topology.Collection.
hxiaodon/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,hxiaodon/seaweedfs,k85169336/seaweedfs,k85169336/seaweedfs