Improve destroy in modules

src/canvas/index.js

@@ -678,8 +678,8 @@ export default () => {
     destroy() {
       canvas.stopListening();
       CanvasView.remove();
-      [canvas, CanvasView].forEach(i => (i = null));
-      c = {};
+      [c, canvas, CanvasView].forEach(i => (i = {}));
+      this.em = {};
     }
   };
 };

src/dom_components/index.js

@@ -740,6 +740,7 @@ export default () => {
       this.clear();
       componentView.remove();
       [c, em, componentsById, component, componentView].forEach(i => (i = {}));
+      this.em = {};
     }
   };
 };

src/keymaps/index.js

@@ -218,6 +218,7 @@ export default () => {
     destroy() {
       this.removeAll();
       [em, config, keymaps].forEach(i => (i = {}));
+      this.em = {};
     }
   };
 };