BreadcrumbControl: Fix errors from wrong indexing

Binding on the result of indexing a list property with an index property
means relying on two independently and asynchronously updated
properties, and that won't work well, especially on boundary value like
last index. Solution is to guard expressions against null to prepare
them for Qt-flavored "eventual consistency" of signals propagation.

Fixes errors in e.g. Plasmoid configuration window when switching tabs:

    file:///usr/lib/qml/org/kde/kirigami.2/private/globaltoolbar/BreadcrumbControl.qml:16:5: Unable to assign [undefined] to QQuickItem*
    file:///usr/lib/qml/org/kde/kirigami.2/private/globaltoolbar/BreadcrumbControl.qml:16:5: Unable to assign [undefined] to QQuickItem*
    file:///usr/lib/qml/org/kde/kirigami.2/private/globaltoolbar/BreadcrumbControl.qml:16:5: Unable to assign [undefined] to QQuickItem*