🚧 Remove non-null assertions

Handle these cases better.

src/components/tree/phyloTree/regression.ts

@@ -15,12 +15,15 @@ export interface Regression {
  * The regression is forced to pass through nodes[0].
  */
 function calculateRegressionThroughRoot(nodes: PhyloNode[]): Regression {
+  if (!nodes[0]) {
+    throw new Error("`nodes` must contain at least one entry to calculate the regression through the root.");
+  }
   const terminalNodes = nodes.filter((d) => !d.n.hasChildren && d.visibility === NODE_VISIBLE);
   const nTips = terminalNodes.length;
   if (nTips===0) {
     return {slope: undefined, intercept: undefined, r2: undefined};
   }
-  const offset = nodes[0]!.x;
+  const offset = nodes[0].x;
   const XY = sum(
     terminalNodes.map((d) => (d.y) * (d.x - offset))
   ) / nTips;