code: protected String findString(String expr, String field, String errorMsg) {
      if (expr == null) {
        throw fieldError(field, errorMsg, null);
      } else {
        return findString(expr);
      }
    }
docs: Evaluates the OGNL stack to find a String value.
  <p/>
  If the given expression is <tt>null</tt/> a error is logged and a <code>RuntimeException</code>
  is thrown constructed with a messaged based on the given field and errorMsg paramter.
  
  @param expr
  OGNL expression.
  @param field
  field name used when throwing <code>RuntimeException</code>.
  @param errorMsg
  error message used when throwing <code>RuntimeException</code> .
  @return the String value found.
  @throws StrutsException
  is thrown in case of expression is <tt>null</tt>.