Fix an issue with focus lost after closing terminal panel

Summary:
After leaving terminal with Ctrl-D or exit commands the input focus isn't set back to the folder view.
The problem appears, because `TerminalPanel::isHiddenInVisibleWindow` returns not what it supposed to return,
since when the terminal process exits, `m_terminal` is set to nullptr.

I moved unwanted checks from it inside `TerminalPanel::dockVisibilityChanged`

This change also exposes a crash in `DolphinMainWindow::slotTerminalPanelVisibilityChanged()`,
which was previously working only by luck.
Now we check whether `m_activeViewContainer` is not null before using it.

BUG: 407979
FIXED-IN: 19.11.80

Test Plan:
1. Open Dolphin
2. Press F4 to open the terminal panel
3. Type exit<Enter> or press Ctrl-D
4. Check current focus widget

Reviewers: #dolphin

Subscribers: ngraham, elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D22420