QmlProfiler: Don't use the canvas context if it isn't ready

The documentation indicates that onPaint can actually be called
while the context isn't ready.

Change-Id: I32d861e016e5e990f3d940a35ec28956902041ba
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

src/plugins/qmlprofiler/qml/Overview.qml

@@ -92,6 +92,8 @@ Canvas {
 
     // ***** slots
     onPaint: {
+        if (context === null)
+            return; // canvas isn't ready
         Plotter.qmlProfilerModelProxy = qmlProfilerModelProxy;
         if (dataReady) {
             Plotter.plot(canvas, context, region);

src/plugins/qmlprofiler/qml/TimeDisplay.qml

@@ -49,6 +49,8 @@ Canvas {
     }
 
     onPaint: {
+        if (context === null)
+            return; // canvas isn't ready
         context.fillStyle = "white";
         context.fillRect(0, 0, width, height);
 

src/plugins/qmlprofiler/qml/TimeMarks.qml

@@ -47,6 +47,8 @@ Canvas {
     onYChanged: requestPaint()
 
     onPaint: {
+        if (context === null)
+            return; // canvas isn't ready
         drawBackgroundBars( context, region );
 
         var totalTime = endTime - startTime;