only destroy children when they are nodes

this._children aren't all CollapseTreeNodes. some of the are regular values

addon/-private/collapse-tree.js

@@ -71,7 +71,11 @@ class CollapseTreeNode extends EmberObject {
   */
   cleanChildren() {
     if (this._children) {
-      this._children.forEach(n => n.destroy());
+      for (let child of this._children) {
+        if (child instanceof CollapseTreeNode) {
+          child.destroy();
+        }
+      }
       this._children = null;
     }
   }
@@ -367,7 +371,9 @@ export default class CollapseTree extends EmberObject.extend(EmberArray) {
   destroy() {
     super.destroy(...arguments);
 
-    this._root.destroy();
+    if (this._root) {
+      this._root.destroy();
+    }
   }
 
   /*

package.json

@@ -24,7 +24,7 @@
   },
   "dependencies": {
     "@addepar/ember-toolbox": "^0.3.2",
-    "@ember-decorators/argument": "^0.8.10",
+    "@ember-decorators/argument": "^0.8.15",
     "@ember-decorators/babel-transforms": "^0.1.1",
     "@html-next/vertical-collection": "1.0.0-beta.10",
     "css-element-queries": "^0.4.0",

tests/unit/-private/collapse-tree-test.js

@@ -1,6 +1,7 @@
 import { module, test } from 'qunit';
 import { A as emberA } from '@ember/array';
 import { get, set } from '@ember/object';
+import { run } from '@ember/runloop';
 
 import CollapseTree from 'ember-table/-private/collapse-tree';
 
@@ -179,4 +180,14 @@ module('Unit | Private | CollapseTree', function() {
       assert.equal(collapseTree.objectAt(i).parents.length, expectedDepth[i]);
     }
   });
+
+  test('works with single level tree', function(assert) {
+    let rows = [{ label: 'A', children: [] }, { label: 'B', children: [] }];
+    let tree = CollapseTree.create({ tree: rows });
+    run(() => {
+      tree.get('length');
+      tree.destroy();
+      assert.ok(true, 'exception not thrown');
+    });
+  });
 });