project: Fix possible crash when trying to delete file

Fixes the following:

#0  0x1 in QStandardItem::parent() const () from /usr/lib/libQt5Gui.so.5
#1  0x2 in KateProjectViewTree::removeFile (...) at kateprojectviewtree.cpp:196
#2  0x3 in operator() (...) at project/kateprojecttreeviewcontextmenu.cpp:176
#3  KateProjectTreeViewContextMenu::exec (...) at kateprojecttreeviewcontextmenu.cpp:202
#4  0x4 in KateProjectViewTree::contextMenuEvent (...) at kateprojectviewtree.cpp:271
#5  0x5 in QWidget::event(QEvent*) () from /usr/lib/libQt5Widgets.so.5
#6  0x6 in QFrame::event(QEvent*) () from /usr/lib/libQt5Widgets.so.5

(cherry picked from commit 90b32f0e)