Function will not run (and shows with red cross in SQL Developer)

Question

I have created the function below by typing into the "Enter SQL Statement" box in the SQL Developer tool and running. I see the message "create or REPLACE FUNCTION Statement Processed". I see the Function in the tree view, but it has a red cross icon next to it, so I guess somethings wrong with it. When I try to run it I get the message "The selected program is in an invalid state for running. Recompile the program and try again." If I right-click the function and compile then I get no errors but the red cross remains and I still can't run it.
What am I doing wrong?
Also, I am planning on supplying a SQL script to customers that will have this function at the top of the script, and then I will use the function throughout the rest of the script to decide whether or not to drop a table before re-creating it. Will that be ok? i.e. will the function be available to the rest of the script, or would the function creation need to be followed by a commit/grant/other?
CREATE OR REPLACE FUNCTION CHECK_TABLE_EXISTS(tableName VARCHAR2)
RETURN BOOLEAN IS
    tableExists NUMBER(1,0);
BEGIN
      SELECT COUNT(*) INTO tableExists FROM user_tables WHERE table_name=tableName;
      IF tableExists = 1 THEN
        RETURN TRUE;
      ELSE
        RETURN FALSE;
      END IF;
END CHECK_TABLE_EXISTS;Thanks,
Paul

Answer 1

Hello Try the same at sqlplus and see how it goes. It should not throw any error messages as I tried the same as a normal user Scott with only (connect, resource) priveleges. You dont have to grant any specific previleges for user_tables.
-Sri
SRI>conn scott/tiger@sri
Connected.
SRI>CREATE OR REPLACE FUNCTION CHECK_TABLE_EXISTS(tableName VARCHAR2)
  2  RETURN BOOLEAN IS
  3      tableExists NUMBER(1,0);
  4  BEGIN
  5        SELECT COUNT(*) INTO tableExists FROM user_tables WHERE table_name=tableName;
  6        IF tableExists = 1 THEN
  7          RETURN TRUE;
  8        ELSE
  9          RETURN FALSE;
10        END IF;
11  END CHECK_TABLE_EXISTS;
12  /
Function created.
SRI>set serverout on
SRI>begin
  2  if check_table_exists('EMP') = TRUE
  3  THEN
  4  dbms_output.put_line('Found');
  5  else
  6  dbms_output.put_line('Not Found');
  7  end if;
  8  end;
  9  /
Found
PL/SQL procedure successfully completed.