Canvas: don't crash when getContext("2d") is called on destruction.

We'd assume that a Window existed when that's not always the case.
For example:

import QtQuick 2.4
import QtQuick.Window 2.2
Window {
    Loader {
        sourceComponent: Canvas {
            Component.onDestruction: getContext("2d")
        }
        Component.onCompleted: active = false
    }
}

Change-Id: I5f30876e21ced5658adfa3bac494fd4196e0a7e3
Task-number: QTBUG-42222
Reviewed-by: Ulf Hermann <ulf.hermann@theqtcompany.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/quick/items/context2d/qquickcanvasitem.cpp b/src/quick/items/context2d/qquickcanvasitem.cpp
index c31b7726c4..4b28517701 100644
--- a/src/quick/items/context2d/qquickcanvasitem.cpp
+++ b/src/quick/items/context2d/qquickcanvasitem.cpp
@@ -1124,6 +1124,9 @@ bool QQuickCanvasItem::createContext(const QString &contextType)
 {
     Q_D(QQuickCanvasItem);
 
+    if (!window())
+        return false;
+
     if (contextType == QLatin1String("2d")) {
         if (d->contextType.compare(QLatin1String("2d"), Qt::CaseInsensitive) != 0)  {
             d->contextType = QLatin1String("2d");