Improve gui mode gateway clean up

This commit improves how we clean up the gateway once the UI
is closed in gui mode. Although it is highly unlikely, it is possible
for a user to run into a NameError when deleting the gateway if a user
enters gui mode but is NOT on macos and successfully exits the gui
without crashing the session (see next below, this requires some playing
around). This is resolved by assigning the gateway to None, ensuring
there is *something* to delete if no gateway object is assigned.

This commit also adds an additional check for a running JVM instance
when in gui mode on non-macos systems. This prevents a
`jpype._core.JVMNotRunning: Java Virtual Machine is not running`
error that is produced when the imagej ui visbility is polled but
is no longer available because the JVM has been shut down via
gui interactions.

src/imagej/__init__.py

@@ -1227,6 +1227,7 @@ def run_callbacks(ij):
     if mode == Mode.GUI:
         # Show the GUI and block.
         global gateway
+        gateway = None
 
         def show_gui_and_run_callbacks():
             global gateway
@@ -1261,10 +1262,11 @@ def show_gui_and_run_callbacks():
             gateway = show_gui_and_run_callbacks()
             # We are responsible for our own blocking.
             # TODO: Poll using something better than ui().isVisible().
-            while gateway.ui().isVisible():
+            while sj.jvm_started() and gateway.ui().isVisible():
                 time.sleep(1)
 
-        return gateway
+        del gateway
+        return None
 
     # HEADLESS or INTERACTIVE mode: create the gateway and return it.
     return run_callbacks(_create_gateway())