Hello, could you be so kind na help me fix this error, please?

******* script runtime error *******
undefined is not a field object: (file 'scripts/main.gsc', line 2074)
if( GetSubStr(self.blocker.targetname, 0, 1) == "wc")
*
called from:
(file 'scripts/main.gsc', line 2008)
zom thread UltraBoss();
*
called from:
(file 'scripts/main.gsc', line 1983)
thread SpawnUltraBoss();
*
called from:
(file 'scripts/main.gsc', line 1810)
level thread KillZombie(owner, attacker);
*
called from:
<removed thread>
started from:
(file 'scripts/main.gsc', line 1679)
self waittill("damage", damage, attacker, point);
*
************************************

******* script runtime error *******
cannot cast undefined to string: (file 'scripts/main.gsc', line 2074)
if( GetSubStr(self.blocker.targetname, 0, 1) == "wc")
*
called from:
(file 'scripts/main.gsc', line 2008)
zom thread UltraBoss();
*
called from:
(file 'scripts/main.gsc', line 1983)
thread SpawnUltraBoss();
*
called from:
(file 'scripts/main.gsc', line 1810)
level thread KillZombie(owner, attacker);
*
called from:
<removed thread>
started from:
(file 'scripts/main.gsc', line 1679)
self waittill("damage", damage, attacker, point);
*
************************************

******* script runtime error *******
pair 'undefined' and 'wc' has unmatching types 'undefined' and 'string': (file 'scripts/main.gsc', line 2074)
if( GetSubStr(self.blocker.targetname, 0, 1) == "wc")
*
called from:
(file 'scripts/main.gsc', line 2008)
zom thread UltraBoss();
*
called from:
(file 'scripts/main.gsc', line 1983)
thread SpawnUltraBoss();
*
called from:
(file 'scripts/main.gsc', line 1810)
level thread KillZombie(owner, attacker);
*
called from:
<removed thread>
started from:
(file 'scripts/main.gsc', line 1679)
self waittill("damage", damage, attacker, point);
*
************************************

******* script runtime error *******
cannot cast undefined to bool: (file 'scripts/main.gsc', line 2074)
if( GetSubStr(self.blocker.targetname, 0, 1) == "wc")
*
called from:
(file 'scripts/main.gsc', line 2008)
zom thread UltraBoss();
*
called from:
(file 'scripts/main.gsc', line 1983)
thread SpawnUltraBoss();
*
called from:
(file 'scripts/main.gsc', line 1810)
level thread KillZombie(owner, attacker);
*
called from:
<removed thread>
started from:
(file 'scripts/main.gsc', line 1679)
self waittill("damage", damage, attacker, point);
*
************************************
dvar set force_difficulty

******* script runtime error *******
pair 'undefined' and '1' has unmatching types 'undefined' and 'int': (file 'scripts/main.gsc', line 2143)
if(self.killed == true)
*
started from:
(file 'scripts/main.gsc', line 2155)
wait 1.0;
*
************************************

******* script runtime error *******
cannot cast undefined to bool: (file 'scripts/main.gsc', line 2143)
if(self.killed == true)
*
started from:
(file 'scripts/main.gsc', line 2155)
wait 1.0;
*
************************************

Thank you!