MobileTextActionsToolBar check if controlRoot is undefined before using it

Summary:
Prevents warning such as

qml/org/kde/plasma/components.3/mobiletextselection/MobileTextActionsToolBar.qml:62: TypeError: Cannot read property 'selectedText' of null
qml/org/kde/plasma/components.3/mobiletextselection/MobileTextActionsToolBar.qml:70: TypeError: Cannot read property 'selectedText' of null
qml/org/kde/plasma/components.3/mobiletextselection/MobileTextActionsToolBar.qml:78: TypeError: Cannot read property 'canPaste' of null

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25164