Crash when requesting user display names for pills

[stefanceriu]

Thread 0 Crashed:
0   Element                       	0x00000001013973a0 Swift runtime failure: Unexpectedly found nil while implicitly unwrapping an Optional value + 0 (PillsFormatter.swift:0)
1   Element                       	0x00000001013973a0 closure #1 in static PillsFormatter.refreshPills(in:with:) + 1004 (PillsFormatter.swift:205)
2   Element                       	0x0000000101397310 closure #1 in static PillsFormatter.refreshPills(in:with:) + 860 (PillsFormatter.swift:205)
3   Element                       	0x0000000101337790 specialized closure #1 in NSAttributedString.vc_enumerateAttribute<A>(_:in:options:using:) + 140
4   Element                       	0x000000010133794c thunk for @callee_guaranteed (@in_guaranteed Any?, @unowned _NSRange, @unowned UnsafeMutablePointer<ObjCBool>) -> () + 20 (<compiler-generated>:0)
5   Element                       	0x00000001013379fc thunk for @escaping @callee_guaranteed (@in_guaranteed Any?, @unowned _NSRange, @unowned UnsafeMutablePointer<ObjCBool>) -> () + 116 (<compiler-generated>:0)
6   Foundation                    	0x00000001a8fdb358 -[NSAttributedString enumerateAttribute:inRange:options:usingBlock:] + 260 (NSAttributedString.m:363)
7   Element                       	0x0000000101425984 specialized NSAttributedString.vc_enumerateAttribute<A>(_:in:options:using:) + 36 (NSAttributedString.swift:39)
8   Element                       	0x0000000101425984 static PillsFormatter.refreshPills(in:with:) + 36 (PillsFormatter.swift:193)
9   Element                       	0x0000000101425984 RoomBubbleCellData.refreshProfilesIfNeeded(_:) + 384 (RoomBubbleCellData.swift:55)
10  Element                       	0x0000000101425a78 @objc RoomBubbleCellData.refreshProfilesIfNeeded(_:) + 52 (<compiler-generated>:0)
11  Element                       	0x0000000100f754f8 -[MXKRoomDataSource refreshProfilesIfNeeded] + 200