[xubuntu-users] cannot close a process

[John R. Sowden]

I tried to open a text file (a foxpro source file with a .prg 
extension).  The default option was thunar, not mousepad.  I got the 
pointer with the circle indicating that its was doing something.  After 
about a minute, I opened the file with mousepad, read it and closed it. 
The circling/pointer is still there.  I ran top in a term, saw thunar at 
the top. got the PID, killed it, said invalid pid. Then I noticed in top 
that thunar was jumping arround AND the pid was changing.  This is new 
for me.  I still have the circling pointer.  How do I close this process 
and possibly, how do I keep this from happening again?  xubuntu ver 
16.04 LTS

John