Fix FlowComponentRenderer._clear() for some browsers

Fix #274

The previous implementation works with Chrome, but causes client-side
exceptions for Firefox and Edge at least when filtering a Grid that
uses ComponentRenderer.

Tested manually that it fixes the issue, since we are not running
Firefox or Edge tests.

flow-data/src/main/resources/META-INF/resources/frontend/flow-component-renderer.html

@@ -74,8 +74,8 @@
     }
 
     _clear() {
-      while (this.hasChildNodes()) {
-        this.removeChild(this.lastChild);
+      while (this.firstChild) {
+        this.removeChild(this.firstChild);
       }
     }