clean context after fragment is destroyed

app/src/main/java/info/nightscout/androidaps/plugins/Careportal/Dialogs/NewNSTreatmentDialog.java

@@ -141,6 +141,12 @@ public class NewNSTreatmentDialog extends DialogFragment implements View.OnClick
         super.onAttach(activity);
     }
 
+    @Override
+    public void onDetach() {
+        super.onDetach();
+        this.context = null;
+    }
+
     @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container,
                              Bundle savedInstanceState) {

app/src/main/java/info/nightscout/androidaps/plugins/Overview/Dialogs/CalibrationDialog.java

@@ -46,6 +46,12 @@ public class CalibrationDialog extends DialogFragment implements View.OnClickLis
         this.context = context;
     }
 
+    @Override
+    public void onDetach() {
+        super.onDetach();
+        this.context = null;
+    }
+
     @Override
     public View onCreateView(LayoutInflater inflater, ViewGroup container,
                              Bundle savedInstanceState) {

app/src/main/java/info/nightscout/androidaps/plugins/Overview/Dialogs/WizardDialog.java

@@ -123,6 +123,12 @@ public class WizardDialog extends DialogFragment implements OnClickListener, Com
         this.context = context;
     }
 
+    @Override
+    public void onDetach() {
+        super.onDetach();
+        this.context = null;
+    }
+
     @Override
     public void onResume() {
         super.onPause();