Fixes a JS error that came in as part of #3760.

ui/javascripts/app/models.js

@@ -132,13 +132,20 @@ App.Key = Ember.Object.extend(Ember.Validations.Mixin, {
   // Boolean if the value is valid
   valueValid: Ember.computed.empty('errors.Value'),
 
-// Escape any user-entered parts that aren't URL-safe, but put slashes back since
-// they are common in keys, and the UI lets users make "folders" by simply adding
-// them to keys.
+  // Escape any user-entered parts that aren't URL-safe, but put slashes back since
+  // they are common in keys, and the UI lets users make "folders" by simply adding
+  // them to keys.
   Key: function(key, value) {
     // setter
     if (arguments.length > 1) {
-      clean = encodeURIComponent(decodeURIComponent(value)).replace(/%2F/g, "/")
+      clean = value
+      try {
+        clean = decodeURIComponent(clean);
+      } catch (e) {
+        // If they've got something that's not valid URL syntax then keep going;
+        // this means that at worst we might end up double escaping some things.
+      }
+      clean = encodeURIComponent(clean).replace(/%2F/g, "/")
       this.set('cleanKey', clean);
       return clean;
     }