ColumnView: fix some critical messages

[jeremypw]

• Wait to ensure slot container window is not null when activating a slot
• Do not unnecessarily truncate miller view on closing (which triggers active signal and causes an error messages)

Not sure if this is related to #2788 or not but the log supplied there indicated one of the critical messages this PR fixes.

[ryonakano]

Is there any reason why it's not content_type != null but _content_type != null?

I'm not familiar with the code here, but this looks a bit strange for me:

        if (content_type != UNKNOWN_CONTENT && _content_type != null) {

[jeremypw]

@ryonakano I think it was because after referencing the property content_type once it is quicker to reference the backing variable _content_type as we know it is up to date. If the first reference returned null we don't need to try to evaluate the content_type again.