Bad GetPoint() return value can cause crash

There appears to be a problem with the second return value from the frame:GetPoint() method. Sometimes it is a valid table, sometimes it is nil, and sometimes it is an invalid table which can crash the game if you attempt to use it to call something like GetName(). The problem seems to be related to dragging the frame.

I have created a test addon to demonstrate the problem. The addon will create a frame and display a button on it. When you click the button it displays the return values from frame:GetPoint().

After reloading the UI, the addon displays the frame:GetPoint() return values correctly. If you click on the button without moving the frame, the return values are again displayed correctly.

The problem starts when you drag the window. If you click the button after you have stopped dragging the window, the second return value from frame:GetPoint() may not be correct. I have seen it be nil or a table value. When it is a table value, it will sometimes crash the game if you attempt to call the GetName() method for it.

On the live server I sometimes get a nil value, and sometimes the game crashes with a fatal exception error. On the beta server, I always get the nil value while in the game, and sometimes when I logout I get the fatal exception error.

These problems occurred while no other addons were enabled.

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This application has encountered a critical error:

ERROR #132 (0x85100084) Fatal Exception
Program: E:\Games\World of Warcraft\WoW.exe
Exception: 0xC0000005 (ACCESS_VIOLATION) at 001B:0070211C

The instruction at "0x0070211C" referenced memory at "0x00000008".
The memory could not be "read".

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This application has encountered a critical error:

ERROR #132 (0x85100084) Fatal Exception
Program: J:\Burning Crusade Closed Beta\WoW.exe
Exception: 0xC0000005 (ACCESS_VIOLATION) at 001B:00790D67

The instruction at "0x00790D67" referenced memory at "0x0000001C".
The memory could not be "read".

------- The Test Addon ------

TestFrame_Name = "MyTestFrame" .. time(); -- Unique frame name each run

function TestFrame_Test()
local point, relativeTo, relativePoint, xOffset, yOffset = TestFrame_Frame:GetPoint(1);

local t = type(relativeTo);
local f = DEFAULT_CHAT_FRAME;

f:AddMessage(" ");
f:AddMessage("Point=" .. point);
f:AddMessage("type(relativeTo)=" .. (t or "nil"));
f:AddMessage("relativePoint=" .. relativePoint);
f:AddMessage("xOffset=" .. xOffset);
f:AddMessage("yOffset=" .. yOffset);

if (relativeTo) then
f:AddMessage("GetName(relativePoint)=" .. relativeTo:GetName());
end
end

function TestFrame_DragStart()
TestFrame_Frame:StartMoving();
end

function TestFrame_DragStop()
TestFrame_Frame:StopMovingOrSizing();
end

function TestFrame_CreateFrame()
local f = CreateFrame("Frame", TestFrame_Name, UIParent);
TestFrame_Frame = f;

f:SetFrameStrata("BACKGROUND");
f:SetWidth(300);
f:SetHeight(128);

f:EnableMouse(1);
f:SetMovable(1);
f:RegisterForDrag("LeftButton");

f:SetScript("OnDragStart", TestFrame_DragStart);
f:SetScript("OnDragStop", TestFrame_DragStop);

local t = f:CreateTexture(nil, "BACKGROUND");
t:SetTexture(1, 1, 1, 1);
t:SetAllPoints(f);

f:ClearAllPoints();
f:SetPoint("CENTER", "UIParent", "CENTER", 0, 0)
f:Show()
end

function TestFrame_CreateButton()
local b = CreateFrame("Button", NIL, TestFrame_Frame, "GameMenuButtonTemplate");
b:SetWidth(180);
b:SetHeight(50);
b:SetScript("OnClick", TestFrame_Test);
b:ClearAllPoints();
b:SetPoint("CENTER", TestFrame_Frame, "CENTER", 0, 0);
b:SetText("Print GetPoint");
b:Show();
end

TestFrame_CreateFrame();
TestFrame_CreateButton();
TestFrame_Test();