make gl3d, gl2d and mapbox .destroy() idempotent

- i.e. not crash when called multiple times

Author: etpinard

src/plots/gl2d/scene2d.js

@@ -356,6 +356,8 @@ proto.handleAnnotations = function() {
 };
 
 proto.destroy = function() {
+    if(!this.glplot) return;
+
     var traces = this.traces;
 
     if(traces) {

src/plots/gl3d/scene.js

@@ -593,6 +593,8 @@ proto.plot = function(sceneData, fullLayout, layout) {
 };
 
 proto.destroy = function() {
+    if(!this.glplot) return;
+
     this.camera.mouseListener.enabled = false;
     this.container.removeEventListener('wheel', this.camera.wheelListener);
     this.camera = this.glplot.camera = null;

src/plots/mapbox/mapbox.js

@@ -454,8 +454,8 @@ proto.destroy = function() {
     if(this.map) {
         this.map.remove();
         this.map = null;
+        this.container.removeChild(this.div);
     }
-    this.container.removeChild(this.div);
 };
 
 proto.toImage = function() {