Check if base element exists when dragging the rotation form (#3986)

# What this PR does

Right now depending on whether the left bar is pinned or not, the
existing base element (this is to determine the dragging margins) query
might fail if it cannot find the expected element. Added a check to see
whether it exists or not.

CHANGELOG.md

@@ -35,6 +35,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Fix template preview by @vadimkerr ([#3937](https://github.com/grafana/oncall/pull/3937))
+- Fix an issue when dragging the rotation form ([#3986](https://github.com/grafana/oncall/pull/3986))
 
 ## v1.3.106 (2024-02-20)
 

grafana-plugin/src/containers/RotationForm/RotationForm.tsx

@@ -716,10 +716,11 @@ export const RotationForm = observer((props: RotationFormProps) => {
       return;
     }
 
+    const scrollBarReferenceElements = document.querySelectorAll<HTMLElement>('.scrollbar-view');
     // top navbar display has 2 scrollbar-view elements (navbar & content)
-    const baseReferenceEl = document.querySelectorAll<HTMLElement>('.scrollbar-view')[1];
-
-    const baseReferenceElRect = baseReferenceEl.getBoundingClientRect();
+    const baseReferenceElRect = (
+      scrollBarReferenceElements.length === 1 ? scrollBarReferenceElements[0] : scrollBarReferenceElements[1]
+    ).getBoundingClientRect();
 
     const { right, bottom } = baseReferenceElRect;