fix(big-number): guard against null colorPicker in transformProps

colorPicker defaults to null when the colour-picker control has never been
set (e.g. a freshly-created Big Number chart). Destructuring it directly
crashes:

  TypeError: Cannot destructure property 'r' of 'colorPicker' as it is null

When colorPicker is null/undefined, mainColor is now left undefined so that
BigNumberViz falls back to its existing BRAND_COLOR default. The ECharts
trendline options fall back to BRAND_COLOR explicitly since they require a
string value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: aminghadersohi

superset-frontend/plugins/plugin-chart-echarts/src/BigNumber/BigNumberWithTrendline/transformProps.ts

@@ -18,6 +18,7 @@
  */
 import { t } from '@apache-superset/core/translation';
 import {
+  BRAND_COLOR,
   extractTimegrain,
   getNumberFormatter,
   NumberFormats,
@@ -140,8 +141,9 @@ export default function transformProps(
   const compareLag = Number(compareLag_) || 0;
   let formattedSubheader = subheader;
 
-  const { r, g, b } = colorPicker;
-  const mainColor = `rgb(${r}, ${g}, ${b})`;
+  const mainColor = colorPicker
+    ? `rgb(${colorPicker.r}, ${colorPicker.g}, ${colorPicker.b})`
+    : undefined;
 
   const xAxisLabel = getXAxisLabel(rawFormData) as string;
   let trendLineData: TimeSeriesDatum[] | undefined;
@@ -290,12 +292,12 @@ export default function transformProps(
             symbol: 'circle',
             symbolSize: 10,
             showSymbol: false,
-            color: mainColor,
+            color: mainColor ?? BRAND_COLOR,
             areaStyle: {
               color: new graphic.LinearGradient(0, 0, 0, 1, [
                 {
                   offset: 0,
-                  color: mainColor,
+                  color: mainColor ?? BRAND_COLOR,
                 },
                 {
                   offset: 1,