Fix crash in VariableChooser

fce83bd9f84883f93829e6ca9eacf098b018a02d in qtbase causes a crash:
setModel creates a new selection model, which calls currentChanged,
which accesses an uninitialized member. Hence, protect the access to it.

Change-Id: I144f40aa4286f1a9edba24519a30c08ff5091f57
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/coreplugin/variablechooser.cpp

@@ -245,7 +245,10 @@ VariableChooserPrivate::VariableChooserPrivate(VariableChooser *parent)
     : q(parent),
       m_lineEdit(0),
       m_textEdit(0),
-      m_plainTextEdit(0)
+      m_plainTextEdit(0),
+      m_iconButton(0),
+      m_variableTree(0),
+      m_variableDescription(0)
 {
     m_defaultDescription = VariableChooser::tr("Select a variable to insert.");
 
@@ -408,6 +411,7 @@ void VariableChooser::addSupportForChildWidgets(QWidget *parent, MacroExpander *
  */
 void VariableChooserPrivate::updateDescription(const QModelIndex &index)
 {
+    if (m_variableDescription)
         m_variableDescription->setText(m_model.data(index, Qt::ToolTipRole).toString());
 }