Shell: Fix race condition where shell might start accepting commands …

…before foreground thread created

src/crystal/main.py

@@ -189,13 +189,6 @@ def _main(args: List[str]) -> None:
         install_to_linux_desktop_environment()
         sys.exit()
 
-    # Start shell if requested
-    if parsed_args.shell:
-        from crystal.shell import Shell
-        shell = Shell()
-    else:
-        shell = None
-
     # Start GUI subsystem
     import wx
     import wx.xml  # required by wx.richtext; use explicit import as hint to py2app
@@ -293,6 +286,13 @@ def _finish_launch(self, filepath: Optional[str]=None) -> None:
     from crystal.util.xthreading import is_quitting, set_foreground_thread
     set_foreground_thread(threading.current_thread())
     try:
+        # Start shell if requested
+        if parsed_args.shell:
+            from crystal.shell import Shell
+            shell = Shell()
+        else:
+            shell = None
+
         app = MyApp(redirect=False)
 
         # Starts tests if requested

src/crystal/shell.py

@@ -17,6 +17,9 @@
 
 class Shell:
     def __init__(self) -> None:
+        if not has_foreground_thread():
+            raise ValueError('Expected there to be a foreground thread when starting a Shell')
+
         # Setup proxy variables for shell
         _Proxy._patch_help()
         self._project_proxy = _Proxy(f'<unset {Project.__module__}.{Project.__name__} proxy>')