Fix empty text

ng2-components/ng2-activiti-analytics/src/components/raphael/raphael-text.component.ts

@@ -41,6 +41,9 @@ export class RaphaelTextDirective extends RaphaelBase implements OnInit {
 
     ngOnInit() {
         console.log(this.elementRef);
+        if (this.text === null || this.text === undefined) {
+            this.text = '';
+        }
         this.draw(this.position, this.text);
     }