Prevent a possible nullptr dereference

Author: bnmguy

Marlin/src/module/ft_motion.cpp

@@ -89,6 +89,8 @@ xyze_long_t FTMotion::steps = { 0 };            // Step count accumulator.
 
 uint32_t FTMotion::interpIdx = 0;               // Index of current data point being interpolated.
 
+uint8_t FTMotion::current_extruder_idx = 0;     // Cached extruder index.
+
 // Shaping variables.
 #if HAS_FTM_SHAPING
   FTMotion::shaping_t FTMotion::shaping = {
@@ -391,6 +393,8 @@ void FTMotion::reset() {
   TERN_(HAS_EXTRUDERS, e_raw_z1 = e_advanced_z1 = 0.0f);
 
   axis_move_end_ti.reset();
+
+  current_extruder_idx = 0;
 }
 
 // Private functions.
@@ -454,6 +458,9 @@ void FTMotion::init() {
 // Load / convert block data from planner to fixed-time control variables.
 void FTMotion::loadBlockData(block_t * const current_block) {
 
+  // Cache the extruder index for this block
+  current_extruder_idx = current_block->extruder;
+
   const float totalLength = current_block->millimeters,
               oneOverLength = 1.0f / totalLength;
 
@@ -721,7 +728,7 @@ void FTMotion::convertToSteps(const uint32_t idx) {
   #if ENABLED(STEPS_ROUNDING)
     #define TOSTEPS(A,B) int32_t(trajMod.A[idx] * planner.settings.axis_steps_per_mm[B] + (trajMod.A[idx] < 0.0f ? -0.5f : 0.5f))
     const xyze_long_t steps_tar = LOGICAL_AXIS_ARRAY(
-      TOSTEPS(e, E_AXIS_N(stepper.current_block->extruder)), // May be eliminated if guaranteed positive.
+      TOSTEPS(e, E_AXIS_N(current_extruder_idx)), // May be eliminated if guaranteed positive.
       TOSTEPS(x, X_AXIS), TOSTEPS(y, Y_AXIS), TOSTEPS(z, Z_AXIS),
       TOSTEPS(i, I_AXIS), TOSTEPS(j, J_AXIS), TOSTEPS(k, K_AXIS),
       TOSTEPS(u, U_AXIS), TOSTEPS(v, V_AXIS), TOSTEPS(w, W_AXIS)
@@ -730,7 +737,7 @@ void FTMotion::convertToSteps(const uint32_t idx) {
   #else
     #define TOSTEPS(A,B) int32_t(trajMod.A[idx] * planner.settings.axis_steps_per_mm[B]) - steps.A
     xyze_long_t delta = LOGICAL_AXIS_ARRAY(
-      TOSTEPS(e, E_AXIS_N(stepper.current_block->extruder)),
+      TOSTEPS(e, E_AXIS_N(current_extruder_idx)),
       TOSTEPS(x, X_AXIS), TOSTEPS(y, Y_AXIS), TOSTEPS(z, Z_AXIS),
       TOSTEPS(i, I_AXIS), TOSTEPS(j, J_AXIS), TOSTEPS(k, K_AXIS),
       TOSTEPS(u, U_AXIS), TOSTEPS(v, V_AXIS), TOSTEPS(w, W_AXIS)

Marlin/src/module/ft_motion.h

@@ -169,6 +169,8 @@ class FTMotion {
 
     static xyze_long_t steps;
 
+    static uint8_t current_extruder_idx;
+
     // Shaping variables.
     #if HAS_FTM_SHAPING