MINOR BUG: SDRangel Satellite Tracker, satellite position does NOT appear in "Polar Chart"

[gvimlag]

MINOR PROBLEM:
Initially in SDRangel Satellite Tracker, the satellite position does NOT appear in the "Polar Chart".
To make the position appear, the user must switch from "Polar Chart" to "Az/El vs Time", then back to "Polar Chart".
After this switching, the satellite position appears in the "Polar Chart".

EXAMPLE:

  rm -rf ~/.config/f4exb                #start with clean configuration
  /opt/install/sdrangel/bin/sdrangel    #clean start SDRangel (no options)
  Add Rx device                         #add SDR device
    RTL-SDR[0] 00000001
    OK
  Add Features                          #add Satellite Tracker (use default ISS satellite)
    Satellite Tracker
    Apply
    Close
  Start/stop tracking                   #start Satellite Tracker (crash occurs when AOS reaches 00:00:00)
  <AOS counts down to 00:00:00>
  <"Polar Chart" does NOT show ISS>
  <Switch from "Polar Chart" to "Az/El vs Time", then back to "Polar Chart">
  <"Polar Chart" now shows ISS>