[EsercentiBaseActivity]: dovrebbe correggere:
Caused by: java.lang.NullPointerException
at it.wm.perdue.Utils.getPreferenceString(Utils.java:121)
at it.wm.perdue.doveusarla.EsercentiListFragment.onCreate(EsercentiListFragment.java:87)
at it.wm.perdue.doveusarla.EsercentiRistoListFragment.onCreate(EsercentiRistoListFragment.java:34)