Fix bug where Applescript 'current window' would crash if there was no current...

Fix bug where Applescript 'current window' would crash if there was no current window because the _underlyingWindow would be nil.

Issue 5899

sources/iTermScriptingWindow.m

@@ -11,7 +11,11 @@
 @implementation iTermScriptingWindow
 + (instancetype)scriptingWindowWithWindow:(NSWindow *)window {
-    return [[[self alloc] initWithObject:window] autorelease];
+    if (window) {
+        return [[[self alloc] initWithObject:window] autorelease];
+    } else {
+        return nil;
+    }
 }
 - (instancetype)initWithObject:(NSWindow *)window {