Having the same issue on Mac OS 10.13.4 with SourceTree 2.7.4. I am able to get it to work long enough to work with it but it will on the next start. There's two things I'm doing to get it to work temporarily. First I delete the SourceTree folder in Application Support.
Then I have to add all the repositories back into SourceTree. That worked yesterday but when I started up today it crashed again. I don't like that solution so much.
Today I tried moving my local repository folder to a new location where SourceTree could not find it. That allowed it to start. Then I moved the folder back to where SourceTree is looking for it, I added the repositories back in. That worked for a few minutes before crashing. I'm thinking that maybe one of the repositories is corrupt?
This all works fine on my Windows 10 machine.
Here's my error log:
Process: Sourcetree 
Version: 2.7.4 (175)
Code Type: X86-64 (Native)
Parent Process: ??? 
Responsible: Sourcetree 
User ID: 501
Date/Time: 2018-06-06 07:53:51.113 -0400
OS Version: Mac OS X 10.13.4 (17E202)
Report Version: 12
Anonymous UUID: 37BC183C-8428-0D62-1531-7D654FEF3BAB
Time Awake Since Boot: 3200 seconds
System Integrity Protection: enabled
Crashed Thread: 12 Dispatch queue: com.apple.root.default-qos
Exception Type: EXC_BAD_INSTRUCTION (SIGILL)
Exception Codes: 0x0000000000000001, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Termination Signal: Illegal instruction: 4
Termination Reason: Namespace SIGNAL, Code 0x4
Terminating Process: exc handler 
This seems to be working...I move my folder of repositories out of where SourceTree expects it, launch SourceTree, SourceTree shows that the repositories are gone, and then move the folder back without first quitting SourceTree. SourceTree then auto updates.
I've quit SourceTree and relaunched a couple times and it worked. I don't have confidence it that lasting for long. It's too buggy for that level of confidence. At least I can work.
We are investigating the crash. There seems to be an issue loading and reading repo avatars. We will soon have an update on this. In the meanwhile, feel free to remove the ImageCache folder under ~/Library/Application Support/SourceTree/
Thanks for your patience!
Sourcetree Mac Developer
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