SourceTree 1.6.3 and upwards I have the same issue. Every operation I perform causes SourceTree to crash with the following exception: Most of the time SourceTree has successfully executed the operation, but the app is closed on the fatal exception...so I keep having to relaunch SourceTree to continue.

2016-02-11T11:01:48: Fatal error:
System.InvalidOperationException: No process is associated with this object.

2016-02-11T11:01:48: Stack trace:
at System.Diagnostics.Process.EnsureState(State state)
at System.Diagnostics.Process.EnsureState(State state)
at System.Diagnostics.Process.GetProcessHandle(Int32 access, Boolean throwIfExited)
at System.Diagnostics.Process.WaitForExit(Int32 milliseconds)
at SourceTree.Utils.RepoProcess.Run()
at SourceTree.Repo.RepoHandlerGit.GetRepoConfig(String path, String name)
at SourceTree.Repo.RepoHandlerGit.GetFlowPrefixes(String path)
at SourceTree.ViewModel.Flow.FlowChoosePanelViewModel.FetchNextAction(String& itemName)
at SourceTree.ViewModel.Flow.FlowChoosePanelViewModel.<>c_DisplayClass8.<Refresh>b_7()
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.Flow.FlowChoosePanelViewModel.<Refresh>d__a.MoveNext()
— 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)
ERROR [2016-02-11 11:04:42,533] [1] [SourceTree.AppRoot] [HandleCrash] - Fatal error
System.InvalidOperationException: No process is associated with this object.
at System.Diagnostics.Process.EnsureState(State state)
at System.Diagnostics.Process.EnsureState(State state)
at System.Diagnostics.Process.GetProcessHandle(Int32 access, Boolean throwIfExited)
at System.Diagnostics.Process.WaitForExit(Int32 milliseconds)
at SourceTree.Utils.RepoProcess.Run() in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\Utils\RepoProcess.cs:line 673
at SourceTree.Repo.RepoHandlerGit.Summary(String path) in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\Repo\RepoHandlerGit.cs:line 1261
at SourceTree.ViewModel.LogViewModel.<>c_DisplayClass119_0.<GetLogRetrievalFunc>b_0() in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\ViewModel\LogViewModel.cs:line 624
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.LogViewModel.<LoadNextLogBatchFrom>d__121.MoveNext() in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\ViewModel\LogViewModel.cs:line 749
— 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)