Initialize chart to null inside 'render()'

* Fixes ZEPPELIN-2253
* When pie chart's key(domain) is changed, this error is logged:
 'Uncaught TypeError: arcs[idx] is not a function at pie.js:358'
* Even if we change key and values, chart remains broken

* Fix: set this.chart to null, which inturn makes this.render to
intialize a new chart constructor

zeppelin-web/src/app/visualization/builtins/visualization-piechart.js

@@ -33,6 +33,8 @@ export default class PiechartVisualization extends Nvd3ChartVisualization {
   };
 
   render(pivot) {
+    // [ZEPPELIN-2253] New chart function will be created each time inside super.render()
+    this.chart = null;
     var d3Data = this.d3DataFromPivot(
       pivot.schema,
       pivot.rows,