private void findProblem(@Nullable final PsiElement expression) {
      final PsiWhiteSpace[] children = PsiTreeUtil.getChildrenOfType(expression, PsiWhiteSpace.class);
      if (children != null) {
        for (PsiWhiteSpace ws : children) {
          if (ws.getText().contains("\\")) {
            registerProblem(ws, PyPsiBundle.message("INSP.unnecessary.backslash.unnecessary.backslash.in.expression"),
                            new RemoveUnnecessaryBackslashQuickFix());
          }
        }
      }
    }
private void findProblem(@Nullable final PsiElement expression)