Qt報錯：reference to non-static member function must be called； did you mean to call it with no argumen

Qt開發，一行代碼報錯：

reference to non-static member function must be called; did you mean to call it with no arguments?

代碼：

if (tbxD.toPlainText().trimmed().length == 0) { ... }
if (tbxM.toPlainText().trimmed().length == 0) { ... } 
if (tbxS.toPlainText().trimmed().length == 0) { ... }      

這幾行代碼看上去沒什麼特别的，就是取了EditText輸入框控件的文字而已，為什麼會提示什麼 non-static呢？

if (tbxD.toPlainText().trimmed().length() == 0) { ... }
if (tbxM.toPlainText().trimmed().length() == 0) { ... } 
if (tbxS.toPlainText().trimmed().length() == 0) { ... }