fix: check if grid column is attached (#2159)

Fixes: #1981
Warranty: Prevent an exception when changing the size after removing columns
Cherrypick: vaadin/web-components#195

Author: tulioag

src/vaadin-grid.html

@@ -508,7 +508,9 @@
         }
 
         if (this._columnTree) {
-          this._columnTree[this._columnTree.length - 1].forEach(c => c.notifyPath && c.notifyPath('_cells.*', c._cells));
+          this._columnTree[this._columnTree.length - 1].forEach(
+            (c) => c.isConnected && c.notifyPath && c.notifyPath('_cells.*', c._cells)
+          );
         }
 
         Polymer.RenderStatus.beforeNextRender(this, () => {

test/column.html

@@ -587,6 +587,15 @@
           expect(column._bodyTemplate).to.eql(template2);
         });
       });
+
+      it('should not throw an exception when size is changed after removing column', () => {
+        expect(grid.size).to.equal(10);
+        expect(column.isConnected).to.be.true;
+        column.parentNode.removeChild(column);
+        expect(column.isConnected).to.be.false;
+        expect(() => (grid.size = 11)).not.to.throw();
+        expect(grid.size).to.equal(11);
+      });
     });
 
     describe('pro host', () => {