Do not crash when pasting content to projects view

KDevelop crashed when pasting content in case a file in the project tree
view was selected. This commit fixes this.

BT:
10 0x00007fb588c9dab4 in qt_assert (assertion=0x7fb562700528
"destItem->folder()", file=0x7fb5627005b0
"/home/krf/devel/src/kdevplatform/plugins/projectmanagerview/projectmanagerviewplugin.cpp",
line=679) at /home/krf/devel/src/qt/src/corelib/global/qglobal.cpp:2013
11 0x00007fb5626f1a7f in ProjectManagerViewPlugin::pasteFromContextMenu
(this=<optimized out>) at
/home/krf/devel/src/kdevplatform/plugins/projectmanagerview/projectmanagerviewplugin.cpp:679
12 0x00007fb5626f68d6 in ProjectManagerView::eventFilter
(this=0x7ee6620, obj=0x7ee3a90, event=<optimized out>) at
/home/krf/devel/src/kdevplatform/plugins/projectmanagerview/projectmanagerview.cpp:204
13 0x00007fb588de52d1 in
QCoreApplicationPrivate::sendThroughObjectEventFilters (this=0x916c10,
receiver=0x7ee3a90, event=0x7fff22889d00) at
/home/krf/devel/src/qt/src/corelib/kernel/qcoreapplication.cpp:1056
(...)

REVIEW: 107233