Refactoring: mark getTopmostElementAtOffset() as not-null

idea/src/org/jetbrains/kotlin/idea/codeInsight/CodeInsightUtils.java

@@ -325,7 +325,7 @@ public class CodeInsightUtils {
         return CharArrayUtil.shiftBackward(document.getCharsSequence(), lineStartOffset, " \t");
     }
 
-    @Nullable
+    @NotNull
     public static PsiElement getTopmostElementAtOffset(@NotNull PsiElement element, int offset) {
         do {
             PsiElement parent = element.getParent();

idea/src/org/jetbrains/kotlin/idea/debugger/breakpoints/breakpointTypeUtils.kt

@@ -103,7 +103,7 @@ fun computeVariants(
     if (mainMethod != null) {
         result.add(kotlinBreakpointType.KotlinLineBreakpointVariant(
                 XSourcePositionImpl.createByElement(mainMethod),
-                CodeInsightUtils.getTopmostElementAtOffset(elementAt, pos.offset) ?: mainMethod))
+                CodeInsightUtils.getTopmostElementAtOffset(elementAt, pos.offset)))
     }
 
     lambdas.forEachIndexed { ordinal, lambda ->

idea/src/org/jetbrains/kotlin/idea/debugger/evaluate/extractFunctionForDebuggerUtil.kt

@@ -206,7 +206,7 @@ private fun getExpressionToAddDebugExpressionBefore(tmpFile: KtFile, contextElem
 
         val elementAtOffset = tmpFile.findElementAt(lineStart) ?: return null
 
-        return CodeInsightUtils.getTopmostElementAtOffset(elementAtOffset, lineStart) ?: elementAtOffset
+        return CodeInsightUtils.getTopmostElementAtOffset(elementAtOffset, lineStart)
     }
 
     fun shouldStop(el: PsiElement?, p: PsiElement?) = p is KtBlockExpression || el is KtDeclaration || el is KtFile