phsl / new-api

Previously the component unmounted the Modal as soon as `showModelTestModal` became
false, preventing Semi UI from running its cleanup routine. This left `body`
stuck with `overflow: hidden`, disabling page scroll after the dialog closed.

Changes made
– Removed the early `return null` and always keep the Modal mounted; visibility
  is now controlled solely via the `visible` prop.
– Introduced a `hasChannel` guard to safely skip data processing/rendering when
  no channel is selected.
– Added defensive checks for table data, footer and title to avoid undefined
  access when the Modal is hidden.

This fix ensures that closing the test-model dialog correctly restores the
page’s scroll behaviour on both desktop and mobile.