Fix with poppler-26.06.0.

From https://gitlab.com/inkscape/inkscape/-/work_items/6210,
1.4.x-Poppler-26.06.0_rev0.patch

Index: src/extension/internal/pdfinput/svg-builder.cpp
--- src/extension/internal/pdfinput/svg-builder.cpp.orig
+++ src/extension/internal/pdfinput/svg-builder.cpp
@@ -384,6 +384,7 @@ static gchar *svgConvertRGBToText(double r, double g, 
     return (gchar *)&tmp;
 }
 
+// for poppler < 26.06.0
 static std::string svgConvertGfxRGB(GfxRGB *color)
 {
     double r = (double)color->r / 65535.0;
@@ -394,6 +395,14 @@ static std::string svgConvertGfxRGB(GfxRGB *color)
 
 std::string SvgBuilder::convertGfxColor(const GfxColor *color, GfxColorSpace *space)
 {
+    if (!color) {
+        return "";
+    }
+    return convertGfxColor(*color, space);
+}
+
+std::string SvgBuilder::convertGfxColor(const GfxColor &color, GfxColorSpace *space)
+{
     std::string icc = "";
     switch (space->getMode()) {
         case csDeviceGray:
@@ -412,14 +421,14 @@ std::string SvgBuilder::convertGfxColor(const GfxColor
     }
 
     GfxRGB rgb;
-    space->getRGB(color, &rgb);
+    space->_POPPLER_GET_RGB(color, &rgb);
     auto rgb_color = svgConvertGfxRGB(&rgb);
 
     if (!icc.empty()) {
         Inkscape::CSSOStringStream icc_color;
         icc_color << rgb_color << " icc-color(" << icc;
         for (int i = 0; i < space->getNComps(); ++i) {
-            icc_color << ", " << colToDbl((*color).c[i]);
+            icc_color << ", " << colToDbl((color).c[i]);
         }
         icc_color << ");";
         return icc_color.str();
@@ -1204,7 +1213,7 @@ gchar *SvgBuilder::_createGradient(GfxShading *shading
 /**
  * \brief Adds a stop with the given properties to the gradient's representation
  */
-void SvgBuilder::_addStopToGradient(Inkscape::XML::Node *gradient, double offset, GfxColor *color, GfxColorSpace *space,
+void SvgBuilder::_addStopToGradient(Inkscape::XML::Node *gradient, double offset, GfxColor &color, GfxColorSpace *space,
                                     double opacity)
 {
     Inkscape::XML::Node *stop = _xml_doc->createElement("svg:stop");
@@ -1214,7 +1223,7 @@ void SvgBuilder::_addStopToGradient(Inkscape::XML::Nod
     if (space->getMode() == csDeviceGray) {
         // This is a transparency mask.
         GfxRGB rgb;
-        space->getRGB(color, &rgb);
+        space->_POPPLER_GET_RGB(color, &rgb);
         double gray = (double)rgb.r / 65535.0;
         gray = CLAMP(gray, 0.0, 1.0);
         os_opacity << gray;
@@ -1255,8 +1264,8 @@ bool SvgBuilder::_addGradientStops(Inkscape::XML::Node
         if (!svgGetShadingColor(shading, 0.0, &stop1) || !svgGetShadingColor(shading, 1.0, &stop2)) {
             return false;
         } else {
-            _addStopToGradient(gradient, 0.0, &stop1, space, 1.0);
-            _addStopToGradient(gradient, 1.0, &stop2, space, 1.0);
+            _addStopToGradient(gradient, 0.0, stop1, space, 1.0);
+            _addStopToGradient(gradient, 1.0, stop2, space, 1.0);
         }
     } else if (type == _POPPLER_FUNCTION_TYPE_STITCHING) {
         auto stitchingFunc = static_cast<_POPPLER_CONST StitchingFunction*>(func);
@@ -1269,7 +1278,7 @@ bool SvgBuilder::_addGradientStops(Inkscape::XML::Node
         // Add stops from all the stitched functions
         GfxColor prev_color, color;
         svgGetShadingColor(shading, bounds[0], &prev_color);
-        _addStopToGradient(gradient, bounds[0], &prev_color, space, 1.0);
+        _addStopToGradient(gradient, bounds[0], prev_color, space, 1.0);
         for ( int i = 0 ; i < num_funcs ; i++ ) {
             svgGetShadingColor(shading, bounds[i + 1], &color);
             // Add stops
@@ -1279,14 +1288,14 @@ bool SvgBuilder::_addGradientStops(Inkscape::XML::Node
                     expE = (bounds[i + 1] - bounds[i])/expE;    // approximate exponential as a single straight line at x=1
                     if (encode[2*i] == 0) {    // normal sequence
                         auto offset = (bounds[i + 1] - expE) / max_bound;
-                        _addStopToGradient(gradient, offset, &prev_color, space, 1.0);
+                        _addStopToGradient(gradient, offset, prev_color, space, 1.0);
                     } else {                   // reflected sequence
                         auto offset = (bounds[i] + expE) / max_bound;
-                        _addStopToGradient(gradient, offset, &color, space, 1.0);
+                        _addStopToGradient(gradient, offset, color, space, 1.0);
                     }
                 }
             }
-            _addStopToGradient(gradient, bounds[i + 1] / max_bound, &color, space, 1.0);
+            _addStopToGradient(gradient, bounds[i + 1] / max_bound, color, space, 1.0);
             prev_color = color;
         }
     } else { // Unsupported function type
