Fix clearing themes

Setting the theme to null for some reason worked in GWT hosted mode
but not when compiled. Fix both the broken call site and the method
itself.

Change-Id: Ib87c9b02e551c39768dddf0e3feb5dae849e585a
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/Themer.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/Themer.java
index 8221668..a532209 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/Themer.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/Themer.java
@@ -45,13 +45,17 @@
   }
 
   public void set(ThemeInfo theme) {
-    set(theme.css() != null ? theme.css() : cssText,
-        theme.header() != null ? theme.header() : headerHtml,
-        theme.footer() != null ? theme.footer() : footerHtml);
+    if (theme != null) {
+      set(theme.css() != null ? theme.css() : cssText,
+          theme.header() != null ? theme.header() : headerHtml,
+          theme.footer() != null ? theme.footer() : footerHtml);
+    } else {
+      set(cssText, headerHtml, footerHtml);
+    }
   }
 
   public void clear() {
-    set(cssText, headerHtml, footerHtml);
+    set(null);
   }
 
   void init(Element css, Element header, Element footer) {
diff --git a/gerrit-gwtui/src/main/java/com/google/gerrit/client/ui/Screen.java b/gerrit-gwtui/src/main/java/com/google/gerrit/client/ui/Screen.java
index ccc6a56..a26db05 100644
--- a/gerrit-gwtui/src/main/java/com/google/gerrit/client/ui/Screen.java
+++ b/gerrit-gwtui/src/main/java/com/google/gerrit/client/ui/Screen.java
@@ -62,7 +62,7 @@
   protected void onUnload() {
     super.onUnload();
     if (setTheme) {
-      Gerrit.THEMER.set(null);
+      Gerrit.THEMER.clear();
     }
   }