tmf.ui : Only update chart if not disposed

tmf.ui : Only update chart if not disposed

To fix a SWT error (widget is disposed) when chart is closed while the
trace is still loading.

Change-Id: I44015f742908395e68c30a3824bcdfb509a07d4c
Signed-off-by: France Lapointe Nguyen <francelap@gmail.com>
Reviewed-on: https://git.eclipse.org/r/49923
Reviewed-by: Hudson CI
Reviewed-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Tested-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>