FIR IDE: precalculate DuplicatedFirSourceElementsException error message

It may be called from non-read action

idea/idea-frontend-fir/idea-fir-low-level-api/src/org/jetbrains/kotlin/idea/fir/low/level/api/FirModuleResolveState.kt

@@ -152,13 +152,11 @@ internal class FirModuleResolveStateImpl(override val sessionProvider: FirProjec
 }
 
 class DuplicatedFirSourceElementsException(
-    private val existingFir: FirElement,
-    private val newFir: FirElement,
-    private val psi: KtElement
+    existingFir: FirElement,
+    newFir: FirElement,
+    psi: KtElement
 ) : IllegalStateException() {
-    override val message: String?
-        get() =
-            """|The PSI element should be used only once as a real PSI source of FirElement,
+    override val message: String? = """|The PSI element should be used only once as a real PSI source of FirElement,
        |the elements ${if (existingFir.source === newFir.source) "HAVE" else "DON'T HAVE"} the same instances of source elements 
        |
        |existing FIR element is $existingFir with text: