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]);
     }