[Kanegasi]

Originally Posted by Panthergaming a custom frame called pvreadyDialog

There we go. What is this frame? Can you please post all the code related to this frame?

Edit: Nevermind, it's PVPReadyDialog (FrameXML\PVPHelper.xml:224). The part generating the error seems to be one of the two OnClick handlers for the enter/leave buttons:

<Button name="$parentEnterBattleButton" inherits="UIPanelButtonTemplate" text="BATTLEFIELD_JOIN" parentKey="enterButton">
    <Size x="115" y="22"/>
    <Anchors>
        <Anchor point="BOTTOMRIGHT" relativePoint="BOTTOM" x="-7" y="25"/>
    </Anchors>
    <Scripts>
        <OnClick>
            if ( AcceptBattlefieldPort(self:GetParent().activeIndex, true) ) then
                if( StaticPopup_Visible( "DEATH" ) ) then
                    StaticPopup_Hide( "DEATH" );
                end
                StaticPopupSpecial_Hide(self:GetParent());
            end
            PlaySound(SOUNDKIT.IG_CHARACTER_INFO_TAB);
        </OnClick>
    </Scripts>
</Button>
<Button name="$parentLeaveQueueButton" inherits="UIPanelButtonTemplate" text="LEAVE_QUEUE" parentKey="leaveButton">
    <Size x="115" y="22"/>
    <Anchors>
        <Anchor point="BOTTOMLEFT" relativePoint="BOTTOM" x="7" y="25"/>
    </Anchors>
    <Scripts>
        <OnClick>
            if ( AcceptBattlefieldPort(self:GetParent().activeIndex, false) ) then
                StaticPopupSpecial_Hide(self:GetParent());
            end
            PlaySound(SOUNDKIT.IG_CHARACTER_INFO_TAB);
        </OnClick>
    </Scripts>
</Button>

It seems that the activeIndex key of the frame may be nil when AcceptBattlefieldPort is called.