Arduino / C++: Problems in combination with arrays and loops

i've got the following problem: ( the code is used to select all gcode-files from an SD-card and save their names into an String-Array).

String file_names = String(printDirectory(root));
String cmd = split(file_names, '<', 13);
unsigned int i = 1;

while (true) {
            cmd = split(file_names, '<', i);
            cmd.trim();
            if (cmd == "" or cmd == NULL or cmd == "//") {
                break;
            } else {
              Serial.println(String(i) + String(cmd));
              //gcode_files[i] = cmd;
              delay(200);
              i++;
            }
          }

Well it runs well, but if i remove the comment in line 8, the output is broken. Same problem with variables if their not declarated as an array.

The variable gcode_files is an global variable of typ String btw.

String gcode_files[] = {};

Im not sure, do i need some pointer?

The "normal" output, without the array is here:

14:59:38.768 -> 1EINS~1.GCO 14:59:38.768 -> 2ZWEI~1.GCO 14:59:38.768 -> 3DREI~1.GCO 14:59:38.768 -> 4VIER~1.GCO 14:59:38.816 -> 5FUENF~1.GCO

But when i add the array into the code, i have the following output:

15:03:53.824 -> 1EINS~1.GCO

and the code stops at this point (sometimes after 2 or 3 loops). So the loop never really ends correctly.

Any ideas?