ESP32 Crash when I add 1 at a variable which has no link with the crash line

Question

I'm doing a pixelstip with esp32 Display TTGO. I did one which work perfectly with 144 leds. Today, I'm doing the same thing with 644 leds.

I used the same code and just modify the NUMPIXELS (Global) variable from 144 to 644. At this moment the card crash. I try to use 143 or 145 and that crash. I've try some random values and that work sometimes.

I install ESP Exception Decoder and find the line which causes my trouble:

ets Jul 29 2019 12:21:46

rst:0xc (SW_CPU_RESET),boot:0x13 (SPI_FAST_FLASH_BOOT)
configsip: 0, SPIWP:0xee
clk_drv:0x00,q_drv:0x00,d_drv:0x00,cs0_drv:0x00,hd_drv:0x00,wp_drv:0x00
mode:DIO, clock div:1
load:0x3fff0018,len:4
load:0x3fff001c,len:1216
ho 0 tail 12 room 4
load:0x40078000,len:10944
load:0x40080400,len:6388
entry 0x400806b4
Start
chipid : 56403 - chiffre : 18 - soustrction : 1

T1: 0, T2: 0, T3: 0, T4: 0, T : 1095SD CARD : ok
test : ok
Guru Meditation Error: Core  1 panic'ed (StoreProhibited). Exception was unhandled.
Core 1 register dump:
PC      : 0x4000c3f5  PS      : 0x00060530  A0      : 0x800dbdab  A1      : 0x3ffb1e80  
A2      : 0x00000093  A3      : 0x3ffb1ef8  A4      : 0x00000006  A5      : 0x0000ff00  
A6      : 0x00ff0000  A7      : 0xff000000  A8      : 0x00000000  A9      : 0x00000093  
A10     : 0x00000064  A11     : 0x00000000  A12     : 0x00000006  A13     : 0xe881b485  
A14     : 0x00000000  A15     : 0x3ffb8cb0  SAR     : 0x00000018  EXCCAUSE: 0x0000001d  
EXCVADDR: 0x00000093  LBEG    : 0x400014fd  LEND    : 0x4000150d  LCOUNT  : 0xffffffff  

ELF file SHA256: 0000000000000000

Backtrace: 0x4000c3f5:0x3ffb1e80 0x400dbda8:0x3ffb1ea0 0x400dbecb:0x3ffb1ec0         0x400d4a14:0x3ffb1ee0 0x400d4dd6:0x3ffb1f40 0x400de18e:0x3ffb1fb0 0x400879be:0x3ffb1fd0


PC: 0x4000c3f5
EXCVADDR: 0x00000093

Decoding stack results
0x400dbda8: String::copy(char const*, unsigned int) at /home/vcxp/.arduino15/packages/esp32/hardware/esp32/1.0.6/cores/esp32/WString.cpp line 212
0x400dbecb: String::operator=(String const&) at /home/vcxp/.arduino15/packages/esp32/hardware/esp32/1.0.6/cores/esp32/WString.cpp line 263
0x400d4a14: getFile(fs::File) at /home/vcxp/Arduino/PixelTube_ESP32/PixelTube_ESP32.ino line 1194
0x400d4dd6: setup() at /home/vcxp/Arduino/PixelTube_ESP32/PixelTube_ESP32.ino line 309
0x400de18e: loopTask(void*) at /home/vcxp/.arduino15/packages/esp32/hardware/esp32/1.0.6/cores/esp32/main.cpp line 18
0x400879be: vPortTaskWrapper at /home/runner/work/esp32-arduino-lib-builder/esp32-arduino-lib-builder/esp-idf/components/freertos/port.c line 143`

Here is the GetFile function, Which read file on SDcard:

 int getFile(File dir) {
    int numTabs = 0;
    String str;
    while (true) {
     File entry =  dir.openNextFile();
     if (! entry) {
      // no more files
      return numTabs;
     }
     if (!entry.isDirectory()) {
      str = (String)entry.name();
      str = str.substring(1, str.length() - 4);
      nf[numTabs] = (str); //the crash line
      numTabs++;
    }
    }
    }

I've open Wstring.cpp to see if my global variable NUMPIXELS can interact and nothing.

I'm stuck! Is somebody has an explication?