Builder: Singleton_registry: return null when construction spec not available

https://fedorahosted.org/freeipa/ticket/3235

install/ui/src/freeipa/_base/Builder.js

@@ -276,6 +276,7 @@ define(['dojo/_base/declare',
                 !(cs.ctor && typeof cs.ctor === 'function')) {
                 throw {
                     error: 'Build error: missing or invalid ctor or factory',
+                    code: 'no-ctor-fac',
                     spec: cs
                 };
             }

install/ui/src/freeipa/_base/Singleton_registry.js

@@ -65,13 +65,12 @@ define(['dojo/_base/declare',
             var obj = this._map[type];
 
             if (!obj) {
-                if (!this.builder) {
-                    throw {
-                        error: 'Object not initialized: missing builder',
-                        ref: this
-                    };
-                }
+                if (!this.builder) return null;
+                try {
                     obj = this._map[type] = this.builder.build(type);
+                } catch (e) {
+                    if (e.code === 'no-ctor-fac') obj = null;
+                }
             }
 
             return obj;