ButterKnife If this view is optional add '@Nullable' (fields) or '@Optional' (methods) annotation

Caused by: java.lang.IllegalStateException: Required view 'btn_login' with ID 2131558535 for method 'onClick' was not found. If this view is optional add '@Nullable' (fields) or '@Optional' (methods) annotation.

在使用ButterKnife时，有时会发生以上错误，这可能是butterknife.bind方法写在setContentView之前了，一般要保证butterknife.bind方法要在setContentView之后。