From the screenshot, the error happen on GetTranslatedString function of WebGrid_Context.js. One of the events that trigger this function is: when AutoFilterSuggestion is enabled (WebGrid - LayoutSettings - AutoFilterSuggestion) and user right-click on WebGrid column header.

The error happen because h is undefined or null. Since h is the parameter passed by following expression (from GetBasicFilterColumns): d.GetString(b + f[e].Name.substring(3))

The above should expression should return something like: "Equals {0}".

In my opinion, the error might happen if ISGetObject("WebGrid1").GetString("ContextMenu/FilterSuggest/ET") returns undefine or null.
*Please note that the "ET" in string "ContextMenu/FilterSuggest/ET" might vary with: ET, NET, Contain, NotContain, LTET, GTET, and so on.

If you are using custom localization for your WebGrid, please ensure that the xml node under "FilterSuggest" is not left empty/blank.

<Localization culture="default" description="Default Language">
  ...
  <ContextMenu>
    ...
    <FilterSuggest>
      <ET>Equals {0}</ET>
      <NET>Does Not Equal {0}</NET>
      <Contain>Contains {0}</Contain>
      <NotContain>Does Not Contain {0}</NotContain>
      <LTET>Less Than or Equal To {0}</LTET>
      <GTET>Greater Than or Equal To {0}</GTET>
      <OnBefore>On or Before {0}</OnBefore>
      <OnAfter>On or After {0}</OnAfter>
      <Checked>Is Checked (True)</Checked>
      <NotChecked>Is Not Checked (False)</NotChecked>
      <Tomorrow>Tomorrow</Tomorrow>
      <Today>Today</Today>
      <Yesterday>Yesterday</Yesterday>
      <NextWeek>Next Week</NextWeek>
      <ThisWeek>This Week</ThisWeek>
      <LastWeek>Last Week</LastWeek>
      <NextMonth>Next Month</NextMonth>
      <ThisMonth>This Month</ThisMonth>
      <LastMonth>Last Month</LastMonth>
      <Past>Past</Past>
      <Future>Future</Future>
      <Blank>Blank</Blank>
    </FilterSuggest>
    ...
  </ContextMenu>
  ...
</Localization>

Hope this helps.

We aren't customizing the localization files.  Yes, the root cause seems to be that if the internal GetString call returns null/undefined, then other functions will fail.  From what I could tell, this issue can occur if the grid's context menu opens before the default.xml localization file is fully loaded.  So it doesn't have the string to use yet.

Mike