Fixed crash in Edit Commands dialog

The Edit Commands dialog would crash when the user tried to change
command properties right after opening the dialog, before selecting any
of the commands.

This change makes sure the widgets are disabled as appropriate when
opening the dialog, when no command is selected yet.

Closes mapeditor#2914

src/tiled/commandsedit.cpp

@@ -73,6 +73,8 @@ CommandsEdit::CommandsEdit(const QVector<Command> &commands, QWidget *parent)
 
     connect(mUi->wdBrowseButton, &QPushButton::clicked,
             this, &CommandsEdit::browseWorkingDirectory);
+
+    updateWidgets(mUi->treeView->currentIndex());
 }
 
 CommandsEdit::~CommandsEdit()
@@ -130,7 +132,7 @@ void CommandsEdit::setWorkingDirectory(const QString &text)
 
 void CommandsEdit::updateWidgets(const QModelIndex &current)
 {
-    bool enable = (current.row() < mModel->rowCount() - 1);
+    bool enable = current.isValid() && (current.row() < mModel->rowCount() - 1);
 
     mUi->saveBox->setEnabled(enable);
     mUi->executableEdit->setEnabled(enable);