Grid headers not displaying in asp.net modal popups

We are in the process of converting Ext grids from Ext 4.x to Ext 5. No problem getting grids to convert once we change over the grid column filtering from a feature to the plugin. But when we attempt to run converted grids that a in an asp.net modal popup, we don't get the grid/column headings to display. Also in IE9 the scroll bars don't work properly, but they do in Firefox and Chrome.

These grids all worked fine in modal popups with Ext 4.x.

Has anyone encountered similar problems, and if so how did you solve this (assuming you did)?

Thank You

Grid headers not displaying in asp.net modal popups