Renamed local variable so it doesn't interfere with the provided argument.

String errorStr should be filled in only when ::preload returns false.
Rename local variable to prevent confusion with member variable.
Fix GuiTreeViewCtrl: we should use the member var, removed local decl.
Fix: Bit shifting of the 32-bit value with a subsequent expansion to the 64-bit type.

We now shift the 64-bit value, which is now treated correctly.