Fix exception when running Sheriff-o-Matic tests.

Tools/GardeningServer/ui/test/ct-results-panel-tests.html

Index: Tools/GardeningServer/ui/test/ct-results-panel-tests.html
diff --git a/Tools/GardeningServer/ui/test/ct-results-panel-tests.html b/Tools/GardeningServer/ui/test/ct-results-panel-tests.html
index 4529aec2ba9f8a609b1bbecf24acbc4e49798d2d..0a2dbd5cca83aa1501196c93519adbed6e9a9055 100644
--- a/Tools/GardeningServer/ui/test/ct-results-panel-tests.html
+++ b/Tools/GardeningServer/ui/test/ct-results-panel-tests.html
@@ -97,12 +97,19 @@ describe('ct-results-panel', function() {
 
   beforeEach(function(done) {
     panel = document.createElement('ct-results-panel');
+    panel.hidden = true;
+    document.body.appendChild(panel);
+
     if (failures)
       panel.failures = failures;
 
     setTimeout(done);
   });
 
+  afterEach(function() {
+    document.body.removeChild(panel);
+  });
+
   describe('empty', function() {
     before(function() {
       failures = undefined;