Fixed NPE in buildProgrammersMenu

Some platforms may not define directly 'build.core' because it may be defined through a custom menu. For example, the arduboy platform has in the boards.txt: [...] menu.core=Core [...] # core # arduboy-homemade.menu.core.arduboy-core=Arduboy optimized core arduboy-homemade.menu.core.arduboy-core.build.core=arduboy arduboy-homemade.menu.core.arduino-core=Standard Arduino core arduboy-homemade.menu.core.arduino-core.build.core=arduino:arduino [...] the build.core is determined only after applying the submenu options.

Author: cmaglie

app/src/processing/app/Base.java

@@ -1706,7 +1706,7 @@ public void rebuildProgrammerMenu() {
  TargetPlatform corePlatform = null;
 
  String core = board.getPreferences().get("build.core");
- if (core.contains(":")) {
+ if (core != null && core.contains(":")) {
  String[] split = core.split(":", 2);
  corePlatform = BaseNoGui.getCurrentTargetPlatformFromPackage(split[0]);
  }