Safe values for InnoDB variables when using FULLTEXT searches
innodb_buffer_pool_size -- This should be set to about 70% of RAM.  But, if the other settings encroach too much into this, then lower it.  Swapping is bad.
innodb_buffer_pool_instances -- Simply set it to innodb_buffer_pool_size / 1G -- rounded down, but at least 1
innodb_ft_cache_size -- big enough for at least one row of text being indexed via FULLTEXT.  Bigger if practical, but don't squeeze out the buffer_pool
innodb_ft_total_cache_size -- If you are [re]building FULLTEXT in only one thread at a time, this is not relevant.
innodb_ft_result_cache_limit -- This is relevant only to SELECTs that involve a FULLTEXT index.  The max is 4G, but don't set it that large unless you have lots of RAM.  Start with, say, 1% of RAM; if you get an error, then raise it.

The settings are not dynamic, so change the config file and restart mysqld.
How much RAM do you have?  What is a typical and the max size of the column(s) in the fulltext index?