this wasn't a boolean attribute

core/src/main/resources/lib/hudson/projectView.jelly

@@ -33,8 +33,8 @@ THE SOFTWARE.
     <st:attribute name="jobBaseUrl" type="String">
       The base URL of all job links. Normally ${rootURL}/
     </st:attribute>
-    <st:attribute name="showViewTabs" use="required" type="boolean">
-      If the caller rendered a view tabes, set this to true so that CSS is adjusted accordingly.
+    <st:attribute name="showViewTabs" use="optional" type="boolean">
+      If the caller rendered a view tabes, set this attribute so that CSS is adjusted accordingly.
     </st:attribute>
     <st:attribute name="views" type="Collection&lt;View>">
       If non-null, render nested views.