VariableChooser: Avoid null pointer access

The code makes an effort to qobject_cast a pointer and check it, yet it
accesses another pointer to do its thing.

If the currentWidget is a QPlainTextEdit, this simply crashes.

Amends: da3b83589b

Change-Id: Ie79fa233b04c392fd5aa72cd512fcd0f0d829659
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/variablechooser.cpp

@@ -469,8 +469,8 @@ void VariableChooserPrivate::updateButtonGeometry()
     int margin = buttonMargin();
     int rightPadding = 0;
     if (const auto scrollArea = qobject_cast<const QAbstractScrollArea*>(current)) {
-        rightPadding = m_textEdit->verticalScrollBar()->isVisible() ?
-                    m_textEdit->verticalScrollBar()->width() : 0;
+        rightPadding = scrollArea->verticalScrollBar()->isVisible() ?
+                    scrollArea->verticalScrollBar()->width() : 0;
     }
     m_iconButton->setGeometry(current->rect().adjusted(
                                   current->width() - (margin + 4), 0,