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