I haven't worked on Photos for a couple of days - that's why I haven't answered your question. But, within 1 minute of starting it up today, here's part of the error message I got when it crashed:
Process: Photos [4296]
Path: /Applications/Photos.app/Contents/MacOS/Photos
Identifier: com.apple.Photos
Version: 1.5 (370.42.0)
Build Info: PhotoApp-370042000000000~1
Code Type: X86-64 (Native)
Parent Process: ??? [1]
Responsible: Photos [4296]
User ID: 501
Date/Time: 2016-04-20 17:17:30.404 -0700
OS Version: Mac OS X 10.11.4 (15E65)
Report Version: 11
Anonymous UUID: F73771C4-A0CD-447D-3A7F-84009E670A9E
Time Awake Since Boot: 38000 seconds
System Integrity Protection: enabled
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x000007fc3bc3ac90
Exception Note: EXC_CORPSE_NOTIFY
VM Regions Near 0x7fc3bc3ac90:
MALLOC_LARGE_REUSABLE 000000018a252000-000000018a5fa000 [ 3744K] rw-/rwx SM=PRV
-->
STACK GUARD 0000700000000000-0000700000001000 [ 4K] ---/rwx SM=NUL stack guard for thread 3
Application Specific Information:
objc_msgSend() selector name: respondsToSelector:
Photo Foundation logging:
2016-04-20 17:05:39.615: Received changes notification alert: <LiModelChangeGroup: 0x7fc3bb057d80> alert flags : Replay Complete (__59-[RKFaceChangesHandler startListeningForChangesFromMarker:]_block_invoke_2:173)
2016-04-20 17:05:41.464: Missing resource of type 3 for version: RDVersion(0x7fc3bb0a85d0) modelId=3540 uuid=ZKTAB7FmTq2opgT1b0VRMw tableName=RKVersion state=persisted,local (-[PAVersionEditSession _fileURLForResourceType:]:1225)
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libobjc.A.dylib 0x00007fff83b604dd objc_msgSend + 29
1 com.apple.PhotoPrintProduct 0x00000001106311fa -[KHTextView tearDown] + 102
2 com.apple.PhotoPrintProduct 0x0000000110630eb6 -[KHTextView resignFirstResponder] + 25
3 com.apple.AppKit 0x00007fff86596433 -[NSWindow _realMakeFirstResponder:] + 228
4 com.apple.AppKit 0x00007fff865962fb -[NSWindow makeFirstResponder:] + 123
5 com.apple.swe.UXKit 0x000000010f7f553f -[_UXWindow makeFirstResponder:] + 49
6 com.apple.PhotoPrintProduct 0x00000001108ed9ed -[KHFrameView setEditing:] + 340
7 com.apple.PhotoPrintProduct 0x00000001108ea9f9 -[KHFrameView _handleClick:] + 1135
8 com.apple.PhotoPrintProduct 0x00000001108db531 -[KHLayoutView _handleClick:] + 467
9 com.apple.PhotoPrintProduct 0x00000001108e9d04 __35-[KHFrameView _updateEventTrackers]_block_invoke + 64
10 com.apple.swe.UXKit 0x000000010f7e30fb -[UXEventTracker setState:] + 272
11 com.apple.swe.UXKit 0x000000010f79a1b9 -[UXClickEventTracker mouseDown:] + 138
12 com.apple.CoreFoundation 0x00007fff8e5bda6c __invoking___ + 140
13 com.apple.CoreFoundation 0x00007fff8e5bd8fe -[NSInvocation invoke] + 286
14 com.apple.CoreFoundation 0x00007fff8e5d8d56 -[NSInvocation invokeWithTarget:] + 54
15 com.apple.swe.UXKit 0x000000010f7903fd _forwardEventToTrackers + 480
16 com.apple.swe.UXKit 0x000000010f7901e3 -[UXView mouseDown:] + 53
17 com.apple.AppKit 0x00007fff86ca2469 -[NSWindow _handleMouseDownEvent:isDelayedEvent:] + 6322
18 com.apple.AppKit 0x00007fff86ca344d -[NSWindow _reallySendEvent:isDelayedEvent:] + 212
19 com.apple.AppKit 0x00007fff866e263d -[NSWindow sendEvent:] + 517
20 com.apple.AppKit 0x00007fff86662b3c -[NSApplication sendEvent:] + 2540
21 com.apple.AppKit 0x00007fff864c9ef6 -[NSApplication run] + 796
22 com.apple.AppKit 0x00007fff8649346c NSApplicationMain + 1176
23 libdyld.dylib 0x00007fff8fceb5ad start + 1
Thread 1:: Dispatch queue: com.apple.libdispatch-manager
0 libsystem_kernel.dylib 0x00007fff940fcefa kevent_qos + 10
1 libdispatch.dylib 0x00007fff8179f165 _dispatch_mgr_invoke + 216
2 libdispatch.dylib 0x00007fff8179edcd _dispatch_mgr_thread + 52