getNextZIndex() {
			// get all open popups from InstanceManager
			const aAllOpenPopups = getPopups();
			const aValidatedPopups = [];
			const aInvalidatedPopups = [];

			aAllOpenPopups.forEach((oOpenPopup) => {
				// check if non-adaptable popup
				var bValid = _aPopupFilters.every(function(fnFilter) {
					return fnFilter(oOpenPopup);
				});
				if (bValid && _aPopupFilters.length > 0) {
					aValidatedPopups.push(oOpenPopup);
				} else {
					aInvalidatedPopups.push(oOpenPopup);
				}
			});

			// get max Z-Index from validated popups
			const iMaxValidatedZIndex = aValidatedPopups.length > 0
				? _max(getZIndexFromPopups(aValidatedPopups))
				: -1;

			// get minimum Z-Index from invalidated popups
			const iMinInvalidatedZIndex = aInvalidatedPopups.length > 0
				? _min(getZIndexFromPopups(aInvalidatedPopups))
				: -1;

			// compare Z-Index of adaptable and non-adaptable popups - the higher one wins
			if (iMaxValidatedZIndex < iMinInvalidatedZIndex) {
				return this._getNextMinZIndex(iMinInvalidatedZIndex);
			}
			return Popup.getNextZIndex();
		}
Calculates the reliable z-index in the current window considering the global BusyIndicator dialog.

Algorithm:
1) When popups are already open on the screen:
     the highest z-index of validated popups is compared with the lowest z-index of invalidated popups.
     The invalidated popups also include any BusyIndicator that might be open.
2) If the invalidated popups have the higher value then the next z-index is first decremented by 10,
     which gives the last popup z-index and then 1 is added to it.
3) After incrementing 1 in Step 2), the resultant z-index value is compared against an array of assigned z-index values
      by the ZIndexManager. Step 3) is repeated as long as it stays under a max value and a unique value is calculated.
      The max value is the next possible popup z-index - 3 (hardcoded by variable Z_INDICES_RESERVED).
            Example: when BusyIndicator has a z-index 100, then available indexes are:
                     91, 92, 93, 94, 95, 96, 97. Indexes 98 & 99 are used by
                     BusyIndicator internally, therefore we can't rely on them. The reason we start from
                     the index 91 is that in sap.ui.core.Popup.getNextZIndex() there is a hardcoded step with
                     a value 10 which means there are only 10 reliable indexes between the opened BusyIndicator
                     and the previous absolutely positioned element on the screen;
4) If no popups are open or if validated popups have a higher z-index,
      then simply the next possible z-index is returned by calling sap.ui.core.Popup.getNextZIndex().

@returns {int} the next available z-index value
@public