added error check--- to fix null pointer exception
Merge branch 'master' of https://github.com/tmzhuang/pvz-reloaded

# By Arzaan Irani
# Via Arzaan Irani
* 'master' of https://github.com/tmzhuang/pvz-reloaded:
  SequenceDiagrams for Undo/Redo
  Wallnut GUI added.
  Added in wallnut image + changed gameframe,plant,plantfactory (implementing wallnut)
  Terraintype, trying to compile..
  Added in Defensiveplant (WallNut), a new plant introduced in level 3. The zombie can bit up to 72 times, until the wallnut is disappeared.