filterOption for <OptGroup> may cause exception "TypeError: Cannot read property 'key' of null"

Actually I encountered this problem when using ant-design, here is the reproduction link:

https://codesandbox.io/s/antd-reproduction-template-t25es.

The cause condition:

1. Use <OptGroup> in <Select>;
2. In the <OptGroup>, add some conditional-generated <Option>, for example

const letters = [{ label: "a", value: "a" }, { label: "b", value: "b" }];

<Select.OptGroup label="L">
          {/** when no letters found, display a disabled message option */}
          {letters.length === 0 && (
            <Select.Option disabled key="_" value="_">
              No letters found.
            </Select.Option>
          )}
          {letters.map(item => (
            <Select.Option key={item.value} value={item.value}>
              {item.label}
            </Select.Option>
          ))}
        </Select.OptGroup>

3. type letter L to fulfill the default filter for this <OptGroup>. It will immediately cause an exception as follow:
   

It seems that when filtered the group, <Select> tries to iterate through each children of the <OptGroup> and get its key. But in this case, the children actually includes a false because of the conditional writing.

Start with the linked CodeSandbox reproduction and trace the default filtering path for a Select containing an OptGroup with a conditional false child. Confirm the exception when typing “L”, then verify that filtering completes without error when the conditional child is absent.