private static String stackElementString(TypeBearer type) {
    if (type == null) {
      return "<invalid>";
    }

    return type.toString();
  }
Gets the string form for a stack element. This is the same as {@code toString()} except that
{@code null} is converted to {@code "<invalid>"}.

@param type {@code null-ok;} the stack element
@return {@code non-null;} the string form
android/android_framework/dex-maker/src/main/java/com/android/dx/cf/code/ExecutionStack.java