robustify pie clean step:

- so that it works in cases where _has isn't defined e.g.
  via Plotly.purge().

Author: etpinard

src/plots/plots.js

@@ -193,7 +193,7 @@ plots.getSubplotIds = function getSubplotIds(layout, type) {
     if(type === 'cartesian' && (!layout._has || !layout._has('cartesian'))) return [];
     if(type === 'gl2d' && (!layout._has || !layout._has('gl2d'))) return [];
     if(type === 'cartesian' || type === 'gl2d') {
-        return Object.keys(layout._plots || {});
+        return Object.keys(layout._plots);
     }
 
     var idRegex = _module.idRegex,

src/traces/pie/base_plot.js

@@ -21,7 +21,10 @@ exports.plot = function(gd) {
 };
 
 exports.clean = function(newFullData, newFullLayout, oldFullData, oldFullLayout) {
-    if(oldFullLayout._hasPie && !newFullLayout._hasPie) {
+    var hadPie = (oldFullLayout._has && oldFullLayout._has('pie'));
+    var hasPie = (newFullLayout._has && newFullLayout._has('pie'));
+
+    if(hadPie && !hasPie) {
         oldFullLayout._pielayer.selectAll('g.trace').remove();
     }
 };