In reply to sumanth291092:
Since we cannot run your code, you should edit your post and put a comment pointing to line 378 where the crash is occurring. The only variable that could have a null reference inside the task t_clk_lf is chip_vif, which seems like it would have been set in the build_phase.
Whatever line has the fatal error, you should verify there is no null handle, and if it is null, find out why.
A couple of other notes about your task t_clk_lf. There id no need to explicitly declare class methods with automatic lifetimes as that is the only lifetimes they are allowed.
You should not be using the disable NAMED_BLOCK statement within a class method. if there are more than one instance of the class, the disable statememt has the potential to disable all instances of the block. Use a disable fork instead.
task t_clk_lf();
fork
begin
#36.571136us;
clk_lf_active=1'b0;
end
begin
@(chip_vif.lf_clk_spy);
clk_lf_active=1'b1;
end
join_any
disable fork;
endtask