remove non null assertion

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

src/main/webapp/app/exercises/shared/plagiarism/plagiarism-split-view/split-pane-header/split-pane-header.component.ts

@@ -157,7 +157,10 @@ export class SplitPaneHeaderComponent implements OnChanges, OnInit, OnDestroy {
     }
 
     triggerMouseEnter(file: FileWithHasMatch, idx: number) {
-        this.dropdownHoverSubject()!.next({ idx: idx, file: file });
+        const subject = this.dropdownHoverSubject();
+        if (subject) {
+            subject.next({ idx, file });
+        }
     }
 
     /**