fix(gui): forbid rename method args in fallback mode (#1558)

jadx-core/src/main/java/jadx/api/JavaVariable.java

@@ -4,6 +4,7 @@ import java.util.Collections;
 import java.util.List;
 
 import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.Nullable;
 
 import jadx.api.metadata.ICodeAnnotation;
 import jadx.api.metadata.annotations.VarNode;
@@ -32,7 +33,7 @@ public class JavaVariable implements JavaNode {
 	}
 
 	@Override
-	public String getName() {
+	public @Nullable String getName() {
 		return varNode.getName();
 	}
 

jadx-gui/src/main/java/jadx/gui/treemodel/JVariable.java

@@ -70,6 +70,6 @@ public class JVariable extends JNode {
 
 	@Override
 	public boolean canRename() {
-		return true;
+		return var.getName() != null;
 	}
 }