fix: don't calculate color for invalid times

there is current unrelated behavior that, when
the planner is clicked on to create a new task,
momentarily, the `planItem` object does not
conform to the same structure as it usually does.
This was causing an error when calculating the background.

src/ui/components/task.svelte

@@ -39,12 +39,12 @@
 
   $: colorScale = chroma.scale([$settings.timelineStartColor, $settings.timelineEndColor]).mode("lab");
 
-  $: backgroundColor = $settings.timelineColored
+  $: backgroundColor = $settings.timelineColored && planItem.startTime
     ? colorScale((planItem.startTime.hour() - $settings.startHour )/ (24-$settings.startHour)).hex()
     : "var(--background-primary)";
 
   let properContrastColors: IContrastColors;
-  $: properContrastColors = $settings.timelineColored
+  $: properContrastColors = $settings.timelineColored && planItem.startTime
     ? getTextColorWithEnoughContrast(backgroundColor)
     : {
       normal: "var(--text-normal)",