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*