To reproduce:

• run the attachment. This will create a hg repository and a patch file by using hg diff
• open the repository in SourceTree & do Apply Patch
• select the file "patch.diff" (which is in the newly created repository) as the patch file
  => SourceTree crashes

(note that doing "hg import patch.diff" works without problems)

The relevant portion of sourcetree.log:

2015-07-12T11:39:12: Fatal error:
System.ArgumentException: Second path fragment must not be a drive or UNC name.
Parameter name: path2

2015-07-12T11:39:12: Stack trace:
at System.IO.Path.InternalCombine(String path1, String path2)
at System.IO.FileSystemEnumerableIterator`1.GetFullSearchString(String fullPath, String searchPattern)
at System.IO.FileSystemEnumerableIterator`1..ctor(String path, String originalUserPath, String searchPattern, SearchOption searchOption, SearchResultHandler`1 resultHandler, Boolean checkHost)
at System.IO.Directory.EnumerateFiles(String path, String searchPattern, SearchOption searchOption)
at SourceTree.ViewModel.ApplyPatchDialogViewModel.AnalysePatchImpl(String patchText, Boolean& isCommit, Boolean& isExtendedDiff, Int32& stripLevel, String& basePath, String& msg) in c:\projects\atlassian.com\dev\bitbucket.org\sourcetreewin\ViewModel\ApplyPatchDialogViewModel.cs:line 432
at SourceTree.ViewModel.ApplyPatchDialogViewModel.<>c_DisplayClass7.<AnalysePatch>b_6() in c:\projects\atlassian.com\dev\bitbucket.org\sourcetreewin\ViewModel\ApplyPatchDialogViewModel.cs:line 250
at System.Threading.Tasks.Task`1.InnerInvoke()
at System.Threading.Tasks.Task.Execute()
— End of stack trace from previous location where exception was thrown —
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at SourceTree.ViewModel.ApplyPatchDialogViewModel.<AnalysePatch>d__9.MoveNext() in c:\projects\atlassian.com\dev\bitbucket.org\sourcetreewin\ViewModel\ApplyPatchDialogViewModel.cs:line 252
— End of stack trace from previous location where exception was thrown —
at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__4(Object state)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)