tmf: Handle 'null' active pages in views

tmf: Handle 'null' active pages in views

There is a possible race condition when launching the program,
between the platform's window and the views' active page.

Use getSite() in views where possible instead of PlatformUI, and
add null checks where this is not possible.

Change-Id: I2f94a9eb44817ac9ea906ac93efcf6863511a6d8
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/8772
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>