Prevent error in destroyChart when chartInstance does not exist (#453)

* Prevent error in destroyChart when chartInstance does not exist

* Update src/index.js

Co-Authored-By: Jeremy Ayerst <jeremyayerst@gmail.com>

Co-authored-by: Jeremy Ayerst <jeremyayerst@gmail.com>

Author: andreme

src/index.js

@@ -255,6 +255,10 @@ class ChartComponent extends React.Component {
   }
 
   destroyChart() {
+    if (!this.chartInstance) {
+          return;
+     }
+
     // Put all of the datasets that have existed in the chart back on the chart
     // so that the metadata associated with this chart get destroyed.
     // This allows the datasets to be used in another chart. This can happen,