code: public static <D, R> R safeGet(Mapping<? super D, R> mapping, D key, R fallback) {
          if (mapping == null) {
              return fallback;
          }
          R val = mapping.get(key);
          if (val == null) {
              return fallback;
          }
          return val;
      }
docs: Safely retrieves a value from a mapping. If the mapping is <code>null</code> or returns a <code>null</code>
  value, the given fallback value is returned.
  
  @param mapping
  the mapping.
  @param key
  the key.
  @param fallback
  the fallback value to return if either the mapping or the originally returned value are
  <code>null</code>.
  
  @return the value returned by the specified mapping, or the fallback value.