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>
QmlProfiler: Properly initialize private members of timeline models

Task-number: QTCREATORBUG-11565
Change-Id: I227fba871beeed79923e794c03a08ff154c9e4f8
Reviewed-by: Kai Koehne <kai.koehne@digia.com>