add @Nullable where possible

app/src/main/java/info/nightscout/androidaps/Services/DataService.java

@@ -6,6 +6,7 @@ import android.content.SharedPreferences;
 import android.os.Bundle;
 import android.os.Handler;
 import android.os.HandlerThread;
+import android.support.annotation.Nullable;
 
 import com.j256.ormlite.dao.Dao;
 import com.j256.ormlite.stmt.PreparedQuery;
@@ -332,6 +333,7 @@ public class DataService extends IntentService {
         editor.commit();
     }
 
+    @Nullable
     public static Treatment findById(String _id) {
         try {
             QueryBuilder<Treatment, String> qb = null;
@@ -355,6 +357,7 @@ public class DataService extends IntentService {
         return null;
     }
 
+    @Nullable
     public static Treatment findByTimeIndex(Long timeIndex) {
         try {
             QueryBuilder<Treatment, String> qb = null;

app/src/main/java/info/nightscout/androidaps/db/DatabaseHelper.java

@@ -8,6 +8,7 @@ import java.util.List;
 
 import android.content.Context;
 import android.database.sqlite.SQLiteDatabase;
+import android.support.annotation.Nullable;
 
 import com.j256.ormlite.android.apptools.OrmLiteSqliteOpenHelper;
 import com.j256.ormlite.dao.Dao;
@@ -109,6 +110,7 @@ public class DatabaseHelper extends OrmLiteSqliteOpenHelper {
     /*
      * Return last BgReading from database or null if db is empty
      */
+    @Nullable
     public BgReading lastBg() {
         List<BgReading> bgList = null;
 
@@ -133,6 +135,7 @@ public class DatabaseHelper extends OrmLiteSqliteOpenHelper {
      * Return bg reading if not old ( <9 min )
      * or null if older
      */
+    @Nullable
     public BgReading actualBg() {
         BgReading lastBg = lastBg();
 
@@ -182,6 +185,7 @@ public class DatabaseHelper extends OrmLiteSqliteOpenHelper {
         }
     }
 
+    @Nullable
     public GlucoseStatus getGlucoseStatusData() {
         GlucoseStatus result = new GlucoseStatus();
         try {

app/src/main/java/info/nightscout/androidaps/plugins/Overview/OverviewFragment.java

@@ -105,6 +105,7 @@ public class OverviewFragment extends Fragment implements PluginBase {
         if (profile != null && lastBG != null && bgView != null) {
             bgView.setText(lastBG.valueToUnitsToString(profile.getUnits()));
             DatabaseHelper.GlucoseStatus glucoseStatus = MainApp.getDbHelper().getGlucoseStatusData();
+            if (glucoseStatus != null)
                 deltaView.setText("Δ " + NSProfile.toUnitsString(glucoseStatus.delta, glucoseStatus.delta * Constants.MGDL_TO_MMOLL, units) + " " + units);
             BgReading.units = profile.getUnits();
         } else

app/src/main/java/info/nightscout/androidaps/tabs/TabPageAdapter.java

@@ -1,6 +1,7 @@
 package info.nightscout.androidaps.tabs;
 
 import android.os.Bundle;
+import android.support.annotation.Nullable;
 import android.support.annotation.PluralsRes;
 import android.support.v4.app.Fragment;
 import android.support.v4.app.FragmentManager;
@@ -24,6 +25,7 @@ public class TabPageAdapter extends FragmentPagerAdapter {
     }
 
     @Override
+    @Nullable
     public Fragment getItem(int position) {
         if (position > registeredTabs)
             return null;