[BUG]: UI gets crashed when user deletes Aggregation field under Series section and when clicked on red 'X' button

[pratibhapandey16]

What is the bug?
When single series/metric available and user deletes aggregation field UI gets crashed and also when user clicks on red 'X' button for Aggregation under Series

How can one reproduce the bug?
Flow 1:
Steps to reproduce the behavior:

1. Go to Observability/Event Analytics/Explorer page
2. Enter the query in PPL query section and select 'Year to date' option from calendar and click on 'Refresh'
3. Navigate to Visualizations page
4. Select Vertical bar chart from chart dropdown
5. Delete Aggregation field under series section in Data Config panel
6. Click on Update chart
7. UI gets crashed.

Flow 2:
Steps to reproduce the behavior:

1. Go to Observability/Event Analytics/Explorer page
2. Enter the query in PPL query section and select 'Year to date' option from calendar and click on 'Refresh'
3. Navigate to Visualizations page
4. Select Vertical bar chart from chart dropdown
5. Click on red 'X' button for Aggregation under Series
6. Click on Update chart
7. UI gets crashed.

What is the expected behavior?
The page should load properly.

Do you have any additional context?
Note: The flow 2 issue is reproducible when all the aggregation are deleted from the Series section

@anirudha @mengweieric
cc: @abasatwar @spattnaik