I’ve been struggling for a couple of months , with this very annoying problem. Whenever I issued a query like

 select CAST(COLLECT(TO_CHAR(100)) AS table_varchar) from dual 

where table_varchar is a type I use defined like

 CREATE OR REPLACE TYPE "TABLE_VARCHAR"   AS TABLE OF  VARCHAR2 

my PL/SQL Developer would always crash ! Today , with a tip from a friend I found the reason ! To correct this problem just go to menu TOOLS > PREFERENCES  and choose your Oracle Home if it is null.

Now, choose apply and the OK. After restarting you PL/SQL Developer the problem will no longer occur ! Hopefully this will save you the trouble of monthly crashes like i had !!