What steps will reproduce the problem?
1. Enable portraits on any frame(player, target, etc) especially 3D.
2. Try to select that frame

I got 2 errors:
1.

Quote:

Message: ...rface\AddOns\ag_Extras\modules\portrait\portrait.lua:92: attempt to index global 'this' (a nil value)
Time: 12/21/10 01:34:07
Count: 10
Stack: ...rface\AddOns\ag_Extras\modules\portrait\portrait.lua:92: in function <...rface\AddOns\ag_Extras\modules\portrait\portrait.lua:92>
[C]: in function `Show'
Interface\FrameXML\SecureStateDriver.lua:83: in function <Interface\FrameXML\SecureStateDriver.lua:73>
Interface\FrameXML\SecureStateDriver.lua:137: in function <Interface\FrameXML\SecureStateDriver.lua:119>

Locals: (*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index global 'this' (a nil value)"

2.

Quote:

Message: ...rface\AddOns\ag_Extras\modules\portrait\portrait.lua:92: attempt to index global 'this' (a nil value)
Time: 12/21/10 01:20:51
Count: 1
Stack: ...rface\AddOns\ag_Extras\modules\portrait\portrait.lua:92: in function <...rface\AddOns\ag_Extras\modules\portrait\portrait.lua:92>
[C]: in function `Show'
...rface\AddOns\ag_Extras\modules\portrait\portrait.lua:125: in function `UpdatePortrait'
...rface\AddOns\ag_Extras\modules\portrait\portrait.lua:54: in function `?'
Interface\AddOns\ag_UnitFrames\ag_UnitClass.lua:121: in function `ExecModuleMethods'
Interface\AddOns\ag_UnitFrames\ag_UnitClass.lua:505: in function `?'
Interface\AddOns\ag_UnitFrames\ag_UnitFrames.lua:211: in function `CallMethodOnUnit'
Interface\AddOns\ag_Options\ag_ModuleOptions.lua:491: in function <Interface\AddOns\ag_Options\ag_ModuleOptions.lua:485>
(tail call): ?
[C]: ?
[string "safecall Dispatcher[2]"]:9: in function <[string "safecall Dispatcher[2]"]:5>
(tail call): ?
...
[string "safecall Dispatcher[3]"]:9: in function <[string "safecall Dispatcher[3]"]:5>
(tail call): ?
...ace\AddOns\ClassTimer\Libs\AceGUI-3.0\AceGUI-3.0.lua:314: in function `Fire'
...ns\libs\AceGUI-3.0\widgets\AceGUIWidget-DropDown.lua:439: in function <...ns\libs\AceGUI-3.0\widgets\AceGUIWidget-DropDown.lua:430>
(tail call): ?
[C]: ?
[string "safecall Dispatcher[3]"]:9: in function <[string "safecall Dispatcher[3]"]:5>
(tail call): ?
...ace\AddOns\ClassTimer\Libs\AceGUI-3.0\AceGUI-3.0.lua:314: in function `Fire'
...s\AceGUI-3.0\widgets\AceGUIWidget-DropDown-Items.lua:345: in function <...s\AceGUI-3.0\widgets\AceGUIWidget-DropDown-Items.lua:335>

Locals: (*temporary) = nil
(*temporary) = nil
(*temporary) = nil
(*temporary) = "attempt to index global 'this' (a nil value)"

But.. frames are displaying normally. This happens only when I trying to select any frame which has a portrait,

What version of the product are you using?
Very current. r719.

Do you have an error log of what happened?
Yes.