Crash During VRML export

[DavidMansolino]

Describe the Bug
Webots crashes during the VRML exportation of the city.wbt world.

Additional context
This is the stack:

#0  0x00000000007b4dee in WbField::value (this=0x0) at vrml/WbField.hpp:87
#1  0x0000000000924b0f in WbPbrAppearance::exportNodeSubNodes (this=0xbf55320, 
    writer=...) at nodes/WbPbrAppearance.cpp:527
#2  0x0000000000906c05 in WbNode::exportNodeContents (this=0xbf55320, 
    writer=...) at vrml/WbNode.cpp:1098
#3  0x0000000000906c5c in WbNode::writeExport (this=0xbf55320, writer=...)
    at vrml/WbNode.cpp:1104
#4  0x0000000000905bcb in WbNode::write (this=0xbf55320, writer=...)
    at vrml/WbNode.cpp:963
#5  0x00000000009937d3 in WbSFNode::write (this=0xbf55280, writer=...)
    at vrml/WbSFNode.cpp:57
#6  0x000000000085ad85 in WbField::write (this=0xbf551a0, writer=...)
    at vrml/WbField.cpp:115
#7  0x0000000000906a7c in WbNode::exportNodeSubNodes (this=0xbf55010, 
    writer=...) at vrml/WbNode.cpp:1079
#8  0x0000000000906c05 in WbNode::exportNodeContents (this=0xbf55010, 
    writer=...) at vrml/WbNode.cpp:1098
#9  0x0000000000906c5c in WbNode::write