Handle invalid user being selected in config

Fixed #218

android_app/app/src/main/java/com/health/openscale/core/OpenScale.java

@@ -183,6 +183,10 @@ public class OpenScale {
             final int selectedUserId = getSelectedScaleUserId();
             if (selectedUserId != -1) {
                 selectedScaleUser = userDAO.get(selectedUserId);
+                if (selectedScaleUser == null) {
+                    selectScaleUser(-1);
+                    throw new Exception("could not find the selected user");
+                }
                 return selectedScaleUser;
             }
         } catch (Exception e) {