In case of using custom plugin, scanning QML takes long time when the project is opened with the last session stored.

How to reproduce
1. Enable "File" -> "Sessions" -> "Manage" -> "Restore last session on startup"
2. Open attached project QCBug24628.zip and open main.qml file, then close the project.
3. Open the project from file system.

Red line appears under imported component as Unknown component.
After a few seconds, the following messages appears then the red line disappears.
"Scanning QML Imports"
"Parsing QML Files"
"Scanning for Tests"

The attached video QCBug24628-video.zip shows the phenomena.

A small project like the sample takes a few seconds but a very large project takes longer and sometimes it looks like scanning never complete.

Note: If the project is opened without using the last session (i.e."Restore last session on startup" is disabled and open from File -> Open ... ), there is no delay.