Cascader: Fix issue where the dropdown wouldn't show (#22045)

* Fix z-index on cascader

* Cleanup

* Remove unused import

packages/grafana-ui/src/components/Cascader/Cascader.tsx

@@ -198,6 +198,10 @@ export class Cascader extends React.PureComponent<CascaderProps, CascaderState>
             value={rcValue}
             fieldNames={{ label: 'label', value: 'value', children: 'items' }}
             expandIcon={null}
+            // Required, otherwise the portal that the popup is shown in will render under other components
+            popupClassName={css`
+              z-index: 9999;
+            `}
           >
             <div className={disableDivFocus}>
               <Input

packages/grafana-ui/src/components/ThresholdsEditor/ThresholdsEditor.test.tsx

 import React, { ChangeEvent } from 'react';
 import { mount } from 'enzyme';
-import { GrafanaThemeType, GrafanaTheme, ThresholdsMode } from '@grafana/data';
+import { GrafanaThemeType, ThresholdsMode } from '@grafana/data';
 import { ThresholdsEditor, Props, thresholdsWithoutKey } from './ThresholdsEditor';
 import { colors } from '../../utils';
 import { mockThemeContext } from '../../themes/ThemeContext';
 
 const setup = (propOverrides?: Partial<Props>) => {
   const props: Props = {
-    theme: { type: GrafanaThemeType.Dark, isDark: true, isLight: false } as GrafanaTheme,
     onChange: jest.fn(),
     thresholds: { mode: ThresholdsMode.Absolute, steps: [] },
   };