Better handling of concurrent prompts issues (thanks billarbor for reporting).

src/ui/Prompts.js

@@ -88,10 +88,11 @@ var Prompts = (() => {
             this.done = () => {};
             winMan.close();
             resolve(this.result);
+            if (promptData === this) {
+              promptData = null;
               if (backlog.length) {
                 winMan.open(backlog.shift());
-            } else {
-              promptData = null;
+              }
             }
           }
         };

src/ui/prompt.js

@@ -5,6 +5,11 @@
     .forEach(p => window[p] = bg[p]);
   let data = Prompts.promptData;
   debug(data);
+  if (!data) {
+    error("Missing promptData");
+    window.close();
+    return;
+  }
   let {title, message, options, checks, buttons} = data.features;
 
   function labelFor(el, text) {