To recreate:
1) In SourceTree, right click on a file that has no Windows file association in the current environment (mine was a Visual Studio file named MailMig.csproj.user).
2) Choose 'Open'

After these steps, SourceTree pops up an error dialog entitled 'Oops!' and asks me to report the bug.
Seems like this should open the Windows dialog to choose an editor program rather than crashing.

.\AppData\Local\Atlassian\SourceTree\sourcetree.log says:
_ERROR [2016-07-07 12:59:49,049] [1] [SourceTree.AppRoot] [HandleCrash] - Fatal error
System.NotImplementedException: HandleGeneralTaskFailure is currently unavailable form here. ---> System.ComponentModel.Win32Exception: No application is associated with the specified file for this operation
at System.Diagnostics.Process.StartWithShellExecuteEx(ProcessStartInfo startInfo)
at System.Diagnostics.Process.Start(ProcessStartInfo startInfo)
at SourceTree.Utils.FileHelper.OpenFile(String filename) in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\SourceTree.Api\Utils\FileHelper.cs:line 394
— End of inner exception stack trace —
at SourceTree.Utils.FileHelper.OpenFile(String filename) in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\SourceTree.Api\Utils\FileHelper.cs:line 399
at SourceTree.ViewModel.RepoTabViewModel.OpenSelection() in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\SourceTree.Api.UI.Wpf\ViewModel\RepoTabViewModel.cs:line 1033
at SourceTree.ViewModel.MainWindowViewModel.OpenSelection() in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\ViewModel\MainWindowViewModel.cs:line 1338
at SourceTree.ViewModel.Utils.CommandSink.ExecuteCommand(ICommand command, Object parameter, Boolean& handled) in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\SourceTree.Api.UI.Wpf\ViewModel\Utils\CommandSink.cs:line 119
at SourceTree.ViewModel.Utils.CommandSinkBinding.<set_CommandSink>b__3_1(Object s, ExecutedRoutedEventArgs e) in C:\projects\bitbucket.org\atlassian\sourcetreewin-prod\SourceTree.Api.UI.Wpf\ViewModel\Utils\CommandSinkBinding.cs:line 47
at System.Windows.Input.CommandBinding.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
at System.Windows.Input.CommandManager.ExecuteCommandBinding(Object sender, ExecutedRoutedEventArgs e, CommandBinding commandBinding)
at System.Windows.Input.CommandManager.FindCommandBinding(CommandBindingCollection commandBindings, Object sender, RoutedEventArgs e, ICommand command, Boolean execute)
at System.Windows.Input.CommandManager.FindCommandBinding(Object sender, RoutedEventArgs e, ICommand command, Boolean execute)
at System.Windows.Input.CommandManager.OnExecuted(Object sender, ExecutedRoutedEventArgs e)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.Input.RoutedCommand.ExecuteImpl(Object parameter, IInputElement target, Boolean userInitiated)
at MS.Internal.Commands.CommandHelpers.CriticalExecuteCommandSource(ICommandSource commandSource, Boolean userInitiated)
at System.Windows.Controls.MenuItem.InvokeClickAfterRender(Object arg)
at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)_

Currently having trouble attaching sourcetree.log, getting "JIRA could not attach the file as there was a missing token. Please try attaching the file again."