bufgix: prevent file data loss due to replacing an open file pointer

partial fix for #5275

wled00/file.cpp

@@ -38,7 +38,7 @@ void closeFile() {
     DEBUGFS_PRINT(F("Close -> "));
     uint32_t s = millis();
   #endif
-  f.close();
+  if (f) f.close(); // only close if we have an open file
   DEBUGFS_PRINTF("took %lu ms\n", millis() - s);
   doCloseFile = false;
 }
@@ -271,6 +271,8 @@ bool writeObjectToFile(const char* file, const char* key, const JsonDocument* co
     s = millis();
   #endif
 
+  if (doCloseFile) closeFile(); // This prevents the loss of file data that is still cached in the File object.
+
   size_t pos = 0;
   char fileName[129]; strncpy_P(fileName, file, 128); fileName[128] = 0; //use PROGMEM safe copy as FS.open() does not
   f = WLED_FS.open(fileName, WLED_FS.exists(fileName) ? "r+" : "w+");