Add check for disposed model before setting the property

Check if this._ exists before processing the set operation.

440326: [AutoSuggest] JavaScript Error when setting property of remote
object and destroying it in the same response
https://bugs.eclipse.org/bugs/show_bug.cgi?id=440326

Change-Id: I2d3573d2772b4abab11f303ff9892383d882f681
Signed-off-by: Ivan Furnadjiev <ivan@eclipsesource.com>